Support for type checking any mock with spec

While working on [TestSlide](https://testslide.readthedocs.io/) (a Python test framework from Facebook), @david-caro, @fabriziocucci and myself found an issue with typeguard integration.

In summary, typeguard checks for types recursively (eg: for `Union`, `Tuple` etc), which is a good thing. However, we got failures when one of these recursive checks hits a mock object.

TestSlide type validation functionality plays nicely with mocks, and can [detect and extract their templates](https://github.com/facebookincubator/TestSlide/blob/0b36ffdb40d471d9e945864647265f0033f2d521/testslide/lib.py#L18-L42) for use in type validation. But, if a mock has no template, no type validation happens ([just like typeguard](https://github.com/agronholm/typeguard/blob/4d5dc398cd67952b8cff6a8b0492076156a0bd7d/typeguard/__init__.py#L556)).

TestSlide's interface for this is [public and extensible](https://github.com/facebookincubator/TestSlide/blob/0b36ffdb40d471d9e945864647265f0033f2d521/testslide/strict_mock.py#L734-L741), so we can add supports for any mock object, including TestSlide's own [StrictMock](https://testslide.readthedocs.io/en/master/strict_mock/index.html).

I cut this [clowny PR](https://github.com/facebookincubator/TestSlide/pull/172) for TestSlide, to add the extra logic to deal with mocks, by patching `typeguard.check_type` and wrapping it, so we can move on temporarily.

@agronholm , I'm willing to cut a PR for typeguard to implement this public interface for dealing with mocks, what do you think? Once available at typeguard, we can drop it from TestSlide.