Implement git commit after save

[tennox]

Fixes browserpass/browserpass-extension#347

[max-baz]

Thanks! I think this is a good first step - ping @erayd for feedback too.

In order to fully implement the feature request, I think we need at least to do the following:

1. Add ability to configure path to git binary, just like we do for gpg - I believe this can be configured in .browserpass.json as well as in extension settings.
2. Correctly deal with situations when password store is not a git repo.
3. Also git commit deleted files (delete action)