Add vim-gitgutter plugin

This plugin shows which lines of a file have been changed in a new
gutter. Right now it's pretty interesting, have to see if there's a big
performance hit as I go.

https://github.com/airblade/vim-gitgutter
Add numeric tab navigation

Recently started playing with tabs instead of having a large number of
mvim windows open. I like navigating iTerm tabs with cmd+1, add this
functionality for mvim tabs.