Move Bundles into a separate file

I've had this in [my own dotfiles] for a while now, and really like having the
separation of what I consider to be code/configuration from what to me feels
more like a Gemfile, in that it lists out all of the external dependencies in
a single place.

[my own dotfiles]: https://github.com/calebthompson/dotfiles/blob/master/vim/vimrc.bundles.symlink