Git::config allows scope definition

It's the same functionality I added in: https://github.com/puppetlabs/puppetlabs-git/pull/31
But after the merge of https://github.com/puppetlabs/puppetlabs-git/pull/32 it needed to be redone.

I know the "spec/defines/git_config_spec.rb" is failing, but it was failing before my pull request as well.