Support Git Environment Credentials

[filipekiss]

Users can fallback to using environment variables to set author and commiter details instead of putting it on their .gitconfig file. I came accross this problem and this is a simple solution that will fallback to the same variables git uses if no e-mail or name is configured.

[Shea690901]

Even though I don't need it myself, having configured those in my global config file: :+1:

[ChrisJStone]

I'm in the process of updating gitflow. If you could kindly resubmit your pull request against my fork here as it appears this fork is no longer being updated.