byteify-strings: add --treat-as-kwargs argument to handle kwargs-like objects This argument will help extensions move to Python 3 as keyword arguments should not be byte-prefixed. Most of the time, code bases will call this object `kwargs`, but other conventions exist like `opts`, so it should make sense to allow for custom names. This is a best effort solution that does minimal static checking; cases like `options = [o for o in ('a', 'b', 'c') if kwargs.get(o)]` and other just as complicated will not be detected.

byteify-strings: add helpers to check for item access or method call These helpers will be used in a future patch, split for ease of review.