Allow reading github token from path

[h7x4]

Hello!

This is an attempt at fixing #238. I had a look into urfave/cli, but it seems like the FilePath option they provide, only allows you to set default values from files, not override functionality.

I'm a little bit unsure about the error logging. The logger seems to depend on the arguments, and the arguments (would) depend on the logger. I ended up just using standard fmt commands because I don't think it's worth the complexity, trying to partially parse arguments to set up a logger and then parse the rest. I could move the action function behind the part where the logger is created, but I think it makes sense to keep it as part of the command validation.

I have not written any go code before, so if something looks weird, don't assume I had my reasons 😄

Closes #238

[h7x4]

On second thoughts, the way this is set up now can cause some confusing behaviour. If you were to write a path to a non-existing file or location, it would just assume the string to be the token itself, and try to use it to authenticate. Maybe I should rethink this a bit

[tboerger]

I got to think about it a little bit, I am also thinking about just replacing urfave/cli.

[h7x4]

Do you plan on letting this PR through first, or should I wait until you've replaced it?

[tboerger]

I need some time to think about the alternative ways, I disliked already the single flag used by the GitHub app, if this gets merged the users could start using it the same way which would result in breaking changes if I change it again to a better solution. Give me some time to come back to this PR.

[tboerger]

Thanks for you effort, I have replaced it by #245 now.