Allow reading GitHub token from path

[h7x4]

Hello!

I'm currently working on packaging this program up for NixOS, which I'll assume you at least have heard of since you have a nix flake in your repo.

Because of how nix works with a world-readable storage, it would make things a bit easier for us if we could pass a file path to the --github.token flag, in a similar fashion to --github.private_key. It would also allow other distros to use systemds LoadCredential to safely store and load the token.

Looking forward to your reply!

[tboerger]

I think urfave/cli even got optical file support, that's something that would be quite easy to add.

[tboerger]

I'm currently settling with something like file://path/to/secret/token.txt as you can see at #245, but it will result in a new major version because I will break some flags.

[tboerger]

I have merged #245 now and the documentation have been updated.