Multiple labels support (withLabels)

[chrisui]

I would like to be able to have mention-bot look for multiple labels instead of just a single label.

Ie. ["rfc", "review"]

I'm happy to submit a pr for this but was after some guidance first. Which one of these (providing you think this is reasonable) approached should I take:

a. Add withLabels and maintain support for withLabel
b. Add withLabels and drop support for withLabel
c. Allow array value on withLabel
d. other...

[vjeux]

Hey, sorry about the long delay. I would go with c. I don't want to do a breaking change for this.

Feel free to send a pull request, thanks!