Add in GPG support

[chrisbarber86]

GitHub now supports GPG: https://github.com/blog/2144-gpg-signature-verification

Add support to use the GPG agent + config to always sign commits to dotfiles.

This commit shows how to set up GPG Agent to avoid signing commits manually: https://gist.github.com/sindresorhus/98add7be608fad6b5376a895e5a59972