Could be cool and likely easy to add optional typechecking from annotations with Typeguard

Typeguard https://github.com/agronholm/typeguard makes it very easy to add annotation-based type checking that uses the full gamut of what `typing` offers, with just a decorator, for example.

Using the type checking decorator with Fire is very cool, but I feel like it would be even better if Fire used it internally to display a more user-friendly message to the user when they give an input that doesn't fit the type of the annotation.

Thanks.

@dbieber