Add support for GitHub

[patcon]

This would change the scope a bit, but seems similarly helpful. Some projects I use store artefacts on GitHub rather than Google Drive. Would be nice to pass through same simple app.

Files can be listed like this:
https://api.github.com/repos/hyphacoop/organizing/contents/informational-interviews/

Then most recent commits can be fetched for each file here:
https://api.github.com/repos/hyphacoop/organizing/commits?path=informational-interviews/2019-02-13-ontario-coop-association.md

[patcon]

So this:

https://gdrive-whats-new-app.herokuapp.com/?folder_url=https://github.com/hyphacoop/organizing/tree/master/informational-interviews

Would redirect to https://github.com/hyphacoop/organizing/blob/master/informational-interviews/2019-02-13-ontario-coop-association.md