add case insensitive string literal comparison

I'd like to be able to have predicates that compare case insensitive strings so that I can have a predicate, `hashtag is i'#lol' and username is i'stuntgoat'` and a document `{'username': 'Stuntgoat', 'hashtag': '#LoL'}` evaluate `True`

If this is on the right track, I'll write some tests.