Add fugitive navigation enhancements

The tree buffer makes it easy to drill down through the directories of
your git repository, but it’s not obvious how you could go up a level to
the parent directory. This new mappings ".." helps.
Add numbers.vim to vundles.list

numbers.vim is a plugin that allows for easy switch between
absolute and relative line numbers.
A convenient shortcut: ,tn (toggle numbers) is provided to
switch between the two modes

Fixes #276