Parse :only
Began to implement :only

Still need to:
- Manually test
- Fix VimApp
Made the Vim App conform to the interface change

I have yet to figure out what the appropriate way of actually
implementing `:only` and `:tabonly` for the Vim App is.

Also, did the manual testing for `:only`.
Merge pull request #1459 from Kazark/master

Implement :only to be the same as :tabonly for now