Set git's difftool to `vimdiff`.

When viewing diffs using git, it will now automatically use `vimdiff`
instead of the default diff tool.
Unbind the arrow keys!

Unbound the arrow keys, forcing use of the (proper) hjkl navigation
instead.
Added `vim-powerline` plugin through Vundle

.vimrc now includes settings (and the vundle declaration) for the
`vim-powerline` plugin.

Whilst the `vim-powerline` plugin is being depreceated in favor of the
python-based `powerline` plugin, I haven't had much success getting
this to work on OS X. Maybe I'll try again in the future ;)
Updated .gitconfig to allow plugins in vim.

When first creating .gitconfig, I decided it would be a good idea to
disallow plugins for vim whenever it is used through git with the
`--noplugin` option. I have now decided that I'd like to see my
powerline status bar when committing - and so I've removed this option.
Removed the delay when hitting `ecs` in insert mode.

When hitting `esc` in insert mode, vim pauses for a short amount of
time (~500ms) to wait and see if you are going to press a key
combination using escape. This means there is a noticeable pause
(especially when using powerline) before vim exits insert mode. As I
don't plan to use any escape-based shortcuts, I have simply disabled
this - allowing for immediate exiting from insert mode.