support warnings in addition to errors on commit

Love the gem @jish!  Planning another pull request to add support for my gem, outlaw, as this makes easier some of the work I wanted to do on it for a while, but this functionality is something I definitely want to support, and rather than adding it to my gem, I thought it made sense here.  Let me know if you have any questions or prefer it to be done any differentyly, but tested it locally and it worked well for me.  Thanks!