Add idea for git configuration API improvements

This idea evolved out of pull request #6 via a mailing list discussion
[1].

[1] http://article.gmane.org/gmane.comp.version-control.git/242952
Add Matthieu as possible co-mentor for this idea.
Peff is also willing to help mentor this project
Merge pull request #7 from git/config-caching

Add idea for git configuration API improvements