Support for ignoring forks

[martincostello]

When running multi-gitter against my personal GitHub account, it finds various forks I have of other people's/organization's repositories.

I'd like to be easily able to apply a change to all repositories that I own which are not forks without having to manually curate a list of repositories.

For example, using a configuration like this:

user:
  - martincostello

ignore-forks: true

[lindell]

Seems like a reasonable option to have 🙂 Although I would call it skip-forks to align with the naming of skip-repo.

It will not be on my prio to implement, but I would be happy to merge a PR. If you want to take a look, you can use "topics" as a reference that was implemented to to long ago, but this should be way easier.

https://github.com/lindell/multi-gitter/pull/320/files

[martincostello]

Included in v0.46.0.