Support tag text objects in xml / htmlmixed mode.

User can use `t` to operate on tag text objects. For example, given the
following html:

```
<div>
  <span>hello world!</span>
</div>
```

If the user's cursor (denoted by █) is inside "hello world!":

```
<div>
  <span>hello█world!</span>
</div>
```

And they enter `dit` (delete inner tag), then the text inside the
enclosing tag is deleted -- the following is the expected result:

```
<div>
  <span></span>
</div>
```

If they enter `dat` (delete around tag), then the surrounding tags are
deleted as well:

```
<div>
</div>
```

This logic depends on the following:

- mode/xml/xml.js
- addon/fold/xml-fold.js
- editor is in htmlmixedmode / xml mode

Caveats

This is _NOT_ a 100% accurate implementation of vim tag text objects.
For example, the following cases noop / are inconsistent with vim
behavior:

- Does not work inside comments:
  ```
  <!-- <div>broken</div> -->
  ```
- Does not work when tags have different cases:
  ```
  <div>broken</div>
  ```
- Does not work when inside a broken tag:
  ```
  <div><brok><en></div>
  ```

This addresses codemirror#3828.

keymap/vim.js

@@ -2069,6 +2069,8 @@
             if (operatorArgs) { operatorArgs.linewise = true; }
             tmp.end.line--;
           }
+        } else if (character === 't') {
+          tmp = expandTagUnderCursor(cm, head, inclusive);
         } else {
           // No text object defined for this, don't move.
           return null;
@@ -3295,6 +3297,49 @@
       return { start: Pos(cur.line, start), end: Pos(cur.line, end) };
     }
 
+    /**
+     * Depends on the following:
+     *
+     * - editor mode should be htmlmixedmode / xml
+     * - mode/xml/xml.js should be loaded
+     * - addon/fold/xml-fold.js should be loaded
+     *
+     * If any of the above requirements are not true, this function noops.
+     *
+     * This is _NOT_ a 100% accurate implementation of vim tag text objects.
+     * The following caveats apply (based off cursory testing, I'm sure there
+     * are other discrepancies):
+     *
+     * - Does not work inside comments:
+     *   ```
+     *   <!-- <div>broken</div> -->
+     *   ```
+     * - Does not work when tags have different cases:
+     *   ```
+     *   <div>broken</DIV>
+     *   ```
+     * - Does not work when cursor is inside a broken tag:
+     *   ```
+     *   <div><brok><en></div>
+     *   ```
+     */
+    function expandTagUnderCursor(cm, head, inclusive) {
+      var cur = head;
+      if (!CodeMirror.findMatchingTag || !CodeMirror.findEnclosingTag) {
+        return { start: cur, end: cur };
+      }
+
+      var tags = CodeMirror.findMatchingTag(cm, head) || CodeMirror.findEnclosingTag(cm, head);
+      if (!tags || !tags.open || !tags.close) {
+        return { start: cur, end: cur };
+      }
+
+      if (inclusive) {
+        return { start: tags.open.from, end: tags.close.to };
+      }
+      return { start: tags.open.to, end: tags.close.from };
+    }
+
     function recordJumpPosition(cm, oldCur, newCur) {
       if (!cursorEqual(oldCur, newCur)) {
         vimGlobalState.jumpList.add(cm, oldCur, newCur);
@@ -3836,7 +3881,7 @@
       return Pos(curr_index.ln, curr_index.pos);
     }
 
-    // TODO: perhaps this finagling of start and end positions belonds
+    // TODO: perhaps this finagling of start and end positions belongs
     // in codemirror/replaceRange?
     function selectCompanionObject(cm, head, symb, inclusive) {
       var cur = head, start, end;

test/index.html

@@ -156,6 +156,7 @@ <h2>Test Suite</h2>
     <script src="../addon/mode/multiplex_test.js"></script>
     <script src="../addon/fold/foldcode.js"></script>
     <script src="../addon/fold/brace-fold.js"></script>
+    <script src="../addon/fold/xml-fold.js"></script>
 
     <script src="emacs_test.js"></script>
     <script src="sql-hint-test.js"></script>

test/vim_test.js

@@ -1317,9 +1317,13 @@ testVim('=', function(cm, vim, helpers) {
   eq(expectedValue, cm.getValue());
 }, { value: '   word1\n  word2\n  word3', indentUnit: 2 });
 
-// Edit tests
-function testEdit(name, before, pos, edit, after) {
+// Edit tests - configureCm is an optional argument that gives caller
+// access to the cm object.
+function testEdit(name, before, pos, edit, after, configureCm) {
   return testVim(name, function(cm, vim, helpers) {
+             if (configureCm) {
+               configureCm(cm);
+             }
              var ch = before.search(pos)
              var line = before.substring(0, ch).split('\n').length - 1;
              if (line) {
@@ -1424,6 +1428,28 @@ testEdit('di>_middle_spc', 'a\t<\n\tbar\n>b', /r/, 'di>', 'a\t<>b');
 testEdit('da<_middle_spc', 'a\t<\n\tbar\n>b', /r/, 'da<', 'a\tb');
 testEdit('da>_middle_spc', 'a\t<\n\tbar\n>b', /r/, 'da>', 'a\tb');
 
+// deleting tag objects
+testEdit('dat_noop', '<outer><inner>hello</inner></outer>', /n/, 'dat', '<outer><inner>hello</inner></outer>');
+testEdit('dat_open_tag', '<outer><inner>hello</inner></outer>', /n/, 'dat', '<outer></outer>', function(cm) {
+  cm.setOption('mode', 'xml');
+});
+testEdit('dat_inside_tag', '<outer><inner>hello</inner></outer>', /l/, 'dat', '<outer></outer>', function(cm) {
+  cm.setOption('mode', 'xml');
+});
+testEdit('dat_close_tag', '<outer><inner>hello</inner></outer>', /\//, 'dat', '<outer></outer>', function(cm) {
+  cm.setOption('mode', 'xml');
+});
+
+testEdit('dit_open_tag', '<outer><inner>hello</inner></outer>', /n/, 'dit', '<outer><inner></inner></outer>', function(cm) {
+  cm.setOption('mode', 'xml');
+});
+testEdit('dit_inside_tag', '<outer><inner>hello</inner></outer>', /l/, 'dit', '<outer><inner></inner></outer>', function(cm) {
+  cm.setOption('mode', 'xml');
+});
+testEdit('dit_close_tag', '<outer><inner>hello</inner></outer>', /\//, 'dit', '<outer><inner></inner></outer>', function(cm) {
+  cm.setOption('mode', 'xml');
+});
+
 function testSelection(name, before, pos, keys, sel) {
   return testVim(name, function(cm, vim, helpers) {
              var ch = before.search(pos)