Add [Exposed=*] to expose on all globals

This patch adds support in WebIDL for declaring an interface available
in all contexts.

Closes #468

index.bs

@@ -6539,6 +6539,17 @@ which form (or forms) it is in:
             <code>[Exposed=(Window,Worker)]</code>
         </td>
     </tr>
+    <tr>
+        <td>
+            <emu-nt><a href="#prod-ExtendedAttributeWildcard">ExtendedAttributeWildcard</a></emu-nt>
+        </td>
+        <td>
+            <dfn id="dfn-xattr-wildcard" for="extended attribute" export>takes a wildcard</dfn>
+        </td>
+        <td>
+            <code>[Exposed=*]</code>
+        </td>
+    </tr>
 
 </table>
 
@@ -6600,6 +6611,7 @@ five forms are allowed.
         "="
         "&gt;"
         "?"
+        "*"
         "ByteString"
         "DOMString"
         "FrozenArray"
@@ -6659,6 +6671,11 @@ five forms are allowed.
         identifier "=" identifier
 </pre>
 
+<pre class="grammar" id="prod-ExtendedAttributeWildcard">
+    ExtendedAttributeWildcard :
+        identifier "=" "*"
+</pre>
+
 <pre class="grammar" id="prod-ExtendedAttributeIdentList">
     ExtendedAttributeIdentList :
         identifier "=" "(" IdentifierList ")"
@@ -8560,13 +8577,33 @@ it indicates that the construct is exposed
 on that particular set of global interfaces.
 
 The [{{Exposed}}] [=extended attribute=] must either
-[=takes an identifier|take an identifier=] or
-[=takes an identifier list|take an identifier list=].
+[=takes an identifier|take an identifier=],
+[=takes an identifier list|take an identifier list=] or
+[=takes a wildcard|take a wildcard=].
 Each of the identifiers mentioned must be a [=global name=] and be unique.
-This list of identifiers is known as the construct's
-<dfn>own exposure set</dfn>.
+
+The <dfn>own exposure set</dfn> is the set of [=global name=]s which
+[{{Exposed}}] is applied to.
+
+If the [{{Exposed}}] [=extended attribute=]
+[=takes an identifier|take an identifier=] or
+[=takes an identifier list|take an identifier list=],
+the [=own exposure set=] is the list of identifiers.
+If the [{{Exposed}}] [=extended attribute=]
+[=takes a wildcard|take a wildcard=], the [=own exposure set=] is the
+set of interfaces which have the [{{Global}}] extended attribute.
+
+<p class="advisement">
+    <code>[Exposed=*]</code> should be used with care. It is only appropriate
+    when an interface does not expose significant new capabilities. If the
+    interface may be restricted or disabled in some environments, it is
+    recommended to list the globals explicitly.
+</p>
 
 <div algorithm>
+*   If the [{{Exposed}}] [=extended attribute=] [=takes a wildcard=], then the
+    [=exposure set=] is the set of all interfaces which have a [{{Global}}]
+    or [{{PrimaryGlobal}}] [=extended attribute=].
 
     To get the <dfn id="dfn-exposure-set" export>exposure set</dfn> of a construct |C|,
     run the following steps: