Vim: round indent to multiples of shiftwidth when using > and <

I think this will help me line things up with less effort.
Vim: disable swapfile messages and splash screen

I think that these two options will make my Vim experience a little
smoother. From the docs:

> A   don't give the "ATTENTION" message when an existing swap file is
>     found.
> I   don't give the intro message when starting Vim |:intro|.
Vim: enable persistent undo

I want to be able to keep my undo history around even after I quit Vim
and come back to a file. I think this will be a useful feature.

I decided to story my undo files in the same place I store my Vim backup
files and swap files.
Vim: ignore .sass-cache, bower_components, node_modules during nav

These three directories often contain lots of files that I usually never
want to navigate into. Ignoring them will help make Command-T start up
faster for the first time and will make it easier to actually find the
file that I want.