[vimrc] Create an undo file to ensure persistant undo
[vimrc] Use relativenumber

Instead of linenumbers, display relative number from current cursor. This allows
us to make very quick motions.

Also, remove `set nu` from vimrc since it overrides relativenumber from the
second buffer onwards.
[vimrc] New Plugin: Nerdtree
[vimrc] Map ,, to ESC.