Also look for dotfiles-local

While trying out the approach outlined in Croaky's latest [blog post], I 
realized that running `rcup` would not also pull updates from my local 
extensions. If you clone your extensions as `dotfiles-local`, this update will
pick them up.

Non-existent directories are ignored by rcup, so it won't cause issues if you
don't have `dotfiles-local`.

[blog post]:
http://robots.thoughtbot.com/manage-team-and-personal-dotfiles-together-with-rcm