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.


In the present implementation the closeTag method is not tested, but it calls a number of methods that manipulate text (they do not access the atom APIs). These latter methods are the ones I've been testing so far.

My understanding is that to comply with your suggestion, it suffices to write a new test to check whether the closeTag method integrates correctly with the editor. Am I right? In case I'm not, since the other methods just manipulate text, I would end up writing directly into the spec all the boilerplate code to retrieve the text from the editor and to insert the text into the editor. This does not sound right to me. Am I missing something?