Add support for pull_request_target event

[JakeChampion]

The pull_request_target event is useful for forks as it will use the upstream repositories workflow file and GitHub will give it a read/write access token.

More info for the event can be found in this blog post by GitHub -- https://github.blog/2020-08-03-github-actions-improvements-for-fork-and-pull-request-workflows/

[michaelbeaumont]

This should solve most of the problems mentioned in #46!
Although as far as I can tell, the pull_request_review event in https://github.com/pascalgn/automerge-action#usage will still be a problem for forks (i.e. we can't automerge on approval)? I suspect an approval triggering a merge is a rather rare case since usually an approval comes before the automerge label is added or a check_suite event triggers the merge. In any case, the approver can manually merge.

[pascalgn]

Thanks for the PR, it's merged now! 👍