Added VIM upgrade task to Rakefile

I can now run `rake upgrade_vim_addons` and it will search my
.vim/ directory recursively looking for '.up' YAML files. For
each file it will run a set of pre, update, and post commands
defined in the update file.

Each addon (or bundle) can have it's own '.up' file (as of now
everything in my .vim/ dir does) and they can all be updated w/
one command.

Easy peasy.

:-)