Automatically add a label to the PRs opened by all-contributors

**Describe the solution you'd like**
I would like that the PRs opened by the bot was automatically labeled with a label of choice.

**Describe alternatives you've considered**
This could be achieved adding a `label` field to the all-contributors JSON configuration file.

**Additional context**
I tried to search in the docs and in the past issues and I didn't find an option to do that, I hope to have not missed it!