add optional language parameter to Sandie#evaluate

I added the ability to set language manually, along with a test for it.

I was going to remove the requirement that a language be set on initialization (`evaluate` would raise an error if no language was set), but I decided not to in case that's not how you want it to behave. I can add it back in though if you'd like.

Interesting and useful project, let me know if there's anything I should change in this PR
Will