Add fish config to this project

Includes the work that was previously done in the [fish-functions][]
repo.

[fish-functions]: https://github.com/ciarand/fish-functions
Add unimpaired and (broken) numbertoggle plugins

Unimpaired is great for navigating through syntastic problems (instead
of :lnext and :lprev), and the numbertoggle plugin used to be great but
is currently broken in vim 7.4. I'll have to fix it at some point.
Add a oneline script to install composer (PHP)