Removes unneeded :runtime! command in .vimrc

From :help load-plugins

This command:

:run! plugin/*.vim

Is a subset of what's automatically run by vim on startup:

:run! plugin/**/*.vim
Adds function to override vundle default directory

When using the dotfiles from a user account that doesn't own the
repository, .vim/bundle isn't writable.  So, allow for the possibility
of using a different vundle bundle directory:

~/alt-vim-bundle

If the alt dir exists, it will be used.  Otherwise the function tries to
determine whether the default can be used.
Exports driver variables (for use in Vim)

Possibly, it'd be better to create a wrapper for vim, but for now, just
exporting the vars is fine (exported as ALL_CAPS, per convention).
Fixes #13 - adds per-driver Vim settings

To add per-driver settings, put them in a file called
`driver-$driver.vim` (e.g. `driver-bhaskell.vim`) somewhere in Vim's
runtimepath (e.g. `$drivers_dir`).
Adds missing autocmd for Vim &rnu win-switching

Not sure why BufWinLeave wasn't included in the second :au if
BufWinEnter was included in the first.
Fixes #9 - makes Vim &rnu settable per-driver
Adds example of per-driver Vim settings
Sets default encodings in vim
Switches from vim-powerline to vim-airline