Allow Lexer to take a custom stream

Right now the Lexer only takes input from `stdin`, but I'd like you to be able to embed it in a program other than an interpreter, like an IDE where it could be fed custom lines.

Also, there should be a `reset` method on the Lexer that allows you to clear the "chambered" currTok, so that if you're doing a bunch of short queries, you're sure that they are treated separately. That, or make all of the maps static and then just destruct the instance. Actually, on second thought this is a much better idea, I'll do this if I get time.