Allow vim setup to be configured locally without impacting everyone

Some of our plugins are used by only a handful of folk. These changes make it easier to make changes only to your local vim installation and not every machine.

## How it works

While there is still `.vimrc` and `.vim/bundle`, this PR adds support for an optional `.vimrc_local` file and `.vim/bundle_local` directory. Anything added to this file or directory will override our default setup.

If this is accepted, I'll try to trim down the list of vim modules to what is truly used by many developers.
I think this should be `~/.vimrc_local` to match the `source` line below.