Add [vim-commentary][1]: easy commenting in & out.

[1]: https://github.com/tpope/vim-commentary
Add [vim-repeat][1] for easy repetition.

[1]: https://github.com/tpope/vim-repeat
Add [vim-airline][1] to provide nice status lines.

This is a nice alternative to [vim-powerline][2], which seems to have
gotten a bit out of hand. (It's now [a big mess of Python][3] and the
original, lighter version is no longer supported.)

[1]: https://github.com/bling/vim-airline
[2]: https://github.com/Lokaltog/vim-powerline
[3]: https://github.com/Lokaltog/powerline