Add multi-column unique constraints support for Model.create_or_get
Could probably tidy up the code a little, and we'll need sign off from @coleifer on the `query` conditional, but assuming those two things are signed off, then LGTM.
 @messense Has this already been merged into master? I don't know. I haven't used peewee for a long time and I think I won't be working on this so I closed it. @coleifer Any thoughts on whether you'd like this in master? No, get or create is evil. One of my biggest regrets