Introduce .vimrc.bundles.default

Hi, I think we should be able to overwrite "g:spf13_bundle_groups" for our forks in  ".vimrc.before.fork" file, so the user can install the fork's default bundles. Current bootstrap.sh uses .vimrc.bundles which does not source ".vimrc.before.fork". I have introduced .vimrc.bundles.default, which sources ".vimrc.before.fork" first, and then ".vimrc.bundles". In this way, we can configure the bundles as our fork needs during the installation.

Please check this out.