Add gist-vim[1] plugin for awesome gist interaction

This allows you to create new gists from a buffer or visual selection,
but what I really like is editing a specific gist right from the editor.

This requires a ~/.gist-vim file with an authentication token:

```.gist-vim
token <GitHub Auth token, generated from web ui>
```

It looks like there are good plugins for other editors, too:

- Emacs: https://github.com/defunkt/gist.el
- Sublime Text: https://sublime.wbond.net/packages/Gist

[1]: https://github.com/mattn/gist-vim