Support for ENV with git info

[wireframe]

When using this package with heroku builds, the .git directory is stripped out from deployments, which means all app versions are reported as something like: 1.0.0+.

Heroku exposes some git metadata to the process ENV which could be used when the .git metadata is not available.
ex:

$ heroku config | egrep heroku
HEROKU_SLUG_COMMIT: 12345678

I'm not necessarily recommending adding this just for heroku (although that would be nice), but I'm sure others have hit similar issue.

[rwjblue]

Seems like a good idea for fallback if .git is not present.

Is there a documentation page where I can read about which environment variables are present?

[wireframe]

Yeah, here's a link to the heroku docs.
https://devcenter.heroku.com/changelog-items/630

Looks like they've renamed the variable from HEROKU_SLUG_COMMIT to SOURCE_VERSION.

[rwjblue]

Awesome. Would love a PR for this if you have time, I'm not sure I'll be able to get to it soon...

[rwjblue]

Still happy to have help from folks on this...