Add select_label option

[benCoomes]

This PR adds the select_label option, which causes combine-pr to ignore any PR that does not have a label matching select_label.

This will be useful in a few ways:

• PRs can be manually marked for combination by adding a the 'select label' value and matching any branch name.
• Dependabot PRs can be combined into groups by target branch or update source. For example, tag dependabot PRs for golang with go-dep and PRs for Docker with docker-dep, then combine go changes together and docker changes together.

Some notes on edge cases:

• Both select_label and ignore_label may be set. In this case, the PR is only combined if it has a label matching select_label and does not have any label matching ignore_label.
• If select_label == ignore_label, and error will be raise because it will be impossible for any PRs to be matched and this is almost certainly not what the developer intended.

[benCoomes]

Would it be worth checking labels before CI or approvals, since label checks don't require additional API calls?

[GrantBirki]

Would it be worth checking labels before CI or approvals, since label checks don't require additional API calls?

I think that would be a great idea! Could save some compute and a bit of time when executing this Action

[GrantBirki]

You can either make the change in this PR or we can merge and you can implement on a fresh PR. Up to you! I reviewed the code and it all looks good to me.

[benCoomes]

Thanks! I'll switch the order of checks in a new PR to keep changes small. 🚀