Add gpg signing support

[SuperBo]

Add preliminary support for gpg signing commit. Now only support create commit with signing.

[calops]

This adds libgpgme.so as a dependency, correct? If so, maybe the README should be updated?

[SuperBo]

@calops , yes, but it adds libgpgme.so as a lazy dependency. If you don't install and don't use gpg siging, no problem is raised.

[calops]

The problem is that if do use gpg signing, the plugin stops being able to commit all of a sudden, without having to enable anything in its configuration. That doesn't make it a lazy dependency in my opinion.

[SuperBo]

Thank you @calops, and sorry for disturbing your workflow 🙇. I updated README, let me know if I missed anything else!

[calops]

No worries 👍
It wasn't hard to figure out the issue anyway since the error message mentions the dependency. But maybe it would be better for the plugin itself to detect if that dependency is present and then prompt a proper warning to the user if they're using gpg signing?

[SuperBo]

@calops, Sure, I will revise all error logging and warning. Need to finish some other features first.