Split rvm/rvm_scripts to allow for other version management

Right now you have to trick blazing some to use another version manager besides RVM.  This makes things more flexible so that you don't have to trick it.

It makes rvm_scripts an alias of env_scripts to keep current configs working.

But if you used rbenv and wanted to source a file for it you'd do this:
env_scripts '/etc/profile.d/rbenv.sh'
in your config and have rbenv work(assuming you have that sh).