Stub anything that looks like a Refit interface

This should resolve #65. `InterfaceStubGenerator` now looks for any interface in a file with `using Refit;` that has at least one method with a `[Get|Head|Post|Put|Delete(.*)]` attribute on it.

I have a couple of concerns about the approach, but I'll comment in-line so it makes more sense.
You could roll down the "using" lists to make sure they reference Refit