Fix broken install command (#470)

Regression from #465.

Before:

```
❯ ../bin/elm-test install truqu/elm-md5
I am having trouble with this argument:

    t

It is supposed to be a <package> value, like one of these:

    elm/html
    elm/http
    elm/svg
    elm/time
```

After:

```
❯ ../bin/elm-test install truqu/elm-md5
Here is my plan:

  Add:
    truqu/elm-md5            1.1.0
    zwilias/elm-utf-tools    2.0.1

Would you like me to update your elm.json accordingly? [Y/n]:
Success!
```

This happened because I thought the first argument of the `.action` callback is always an array. Wrong! The signature depends on the `.command` syntax provided:

```js
program
  .command('name <one> <two> [three] [rest...]')
  .action((one: string, two: string, three: string | undefined, rest: Array<string>, cmd: Command) => {
    // Do stuff
  });
```

For the install command we have:

```js
program
  .command('install <package>')
  .action((packageName: string, cmd: Command) => {})
```

Before we tried to destructure the first argument (thinking it was an array), so we only got the first letter of the package name.

Unfortunately our test suite doesn’t catch this.

lib/elm-test.js

@@ -462,7 +462,7 @@ function main() {
     .description(
       'Like `elm install package`, except it installs to "test-dependencies" in your elm.json'
     )
-    .action(([packageName], cmd) => {
+    .action((packageName, cmd) => {
       if (cmd.args.length > 1) {
         // Unfortunately commander is very permissive about extra arguments. Therefore,
         // we manually check for excessive arguments.

tests/flags.js

@@ -150,6 +150,12 @@ describe('flags', () => {
       assert(!runResult.stdout.includes('FINDME'));
       assert(!runResult.stderr.includes('FINDMETWICE'));
     }).timeout(60000);
+
+    it('should exit with success if package already installed', () => {
+      const runResult = execElmTest(['install', 'elm-explorations/test']);
+      console.log(runResult);
+      assert.strictEqual(runResult.status, 0);
+    }).timeout(60000);
   });
 
   describe('elm-test make', () => {