Option to allow merging from forks

[IstoraMandiri]

In #6, all PRs from forked repos are ignored.

It is understandable to ignore forked repos by default, but in the following case, shouldn't this be allowed?

If the target branch is protected and requires maintainer approval(s) for merging, it can be assumed that the PR, once approved, is safe to merge and should be allowed to be scheduled whether or not it's from a forked repo.

[gr2m]

I'm still afraid this might cause unwanted merges.

I'd be okay to turn this into an option that makes explicit how dangerous it is, something like a dangerously_enable_merges_from_forks option, and even then I'd check if branch protections are enabled.

If you or someone else wants to work on it, make sure to update docs and add tests