Adding "passable_boolean" scope with tests.

I mentioned in issue #13 that I wasn't crazy about my own implementation of this, but I'm putting it up here so it can be discussed and tweaked upon and improved (because hey, it's been 3 years)!

This adds the ability to pass true/false into a controller scope that has a model scope accepting the boolean on the backend.

Once it's settled on how to go about this, I'll update my PR with the appropriate clauses in the readme.