add ability to define custom mocks for same type (example, Promises)

[uittorio]

With the current implementation of the library to mock a promise with your special promise you need to

const mockManuallyEdited = createMock<{ promise: Promise<string>; }>();
mockManuallyEdited.promise = new CustomFakePromise();
(mockManuallyEdited.promise as CustomFakePromise<string>).resolve('data');

It would be nice to have the ability to define a mock once and it will be used for that type all the time.

declare function alwaysUseCustomMock<T extends object, U extends T>(factory: () => U): <T2>(cb: (mock: T2) => T) => Extension<T2, U>;
// usage
alwaysUseCustomMock<Promise<any>, CustomFakePromise<any>>(() => new CustomFakePromise());