Add ability to specify Git metadata information

[chshersh]

In the --version option, it's often desired to also provide the git commit hash version. The difficulty here is that Haskell packages can be installed from Hackage, not directly from Git, so applications need to somehow embed git metadata.

Current idea is to provide utlitlies for the application to acquire git version in the release stage but not sure about the best way to do this.

Any ideas on how to provide such an utility are appreciated!