Command to close currently unclosed tags

I've added a command to auto-close tags and bound it to cmd-alt-. .

For instance:
```html
<html>
  <head>
  </head>
  <body>
    <p>By selecting the command (or activating the cmd-alt-. key binding) 
      <a href="some url">in this position, the command would close the "a" tag;  pressing it 
           again, it would close the "p" tag; pressing it a third time nothing would happen (all 
           other tags are closed in the remaining of the document).
</body>
</html>
```

The command works by  identifying the last opened tag that has not yet been closed (at the cursor position). Than checks if in the  part of the html document following the cursor  the found tag is closed. If a closing tag is not found, it then it insert the closing tag at the current cursor position.


Thanks for adding specs, but usually we prefer them at a level that sets text and a cursor on an editor, then triggers the command on the editor view, then validates the editor's text and cursor position are what is expected.

That way we know the code path of someone triggering the command is actually tested and does not regress.