Look in label title for merge type

[mfix22]

By default we have a 'merge when passing' label.

1. If users config other labels to emit the merge action, we should look for squash in the title to change the merge method to squash (same goes for rebase)
2. Maybe we should also enable a default squash when passing and rebase when passing config for users, but don't create those labels for them. That way they can just edit the label from merge -> squash and everything will work out-of-the-box

[mfix22]

@raboid might want to use something like https://www.npmjs.com/package/async-retry to simplify the retry logic between merge/squash/rebase.

https://github.com/dawnlabs/ranger/blob/master/src/pull/labeled.js#L85-L114