Provide mock implementations

[jgornick]

Instead of having to modify mock implementations manually, it might be nice to allow an argument as the first argument in mock<...>({ ... }) which defines mock implementations/values for the instance:

interface ResourcesApi {
    get(id: string): Promise<any>
}

interface Api {
    resources: ResourcesApi
}

const api = mock<Api>({
  resources: {
    get: true // equivalent to api.resources.get.mockReturnValue(true)
    // or get: () => true which is equivalent to api.resources.get.mockImplementation(() => true)
  }
})

// instead of ...
// 

Refer to #3 for the nested support.

Thoughts?

[marchaos]

Hey.

Looks good. I now have this working. I have used DeepPartial so that you can provide incomplete implementations for the top level type and lower levels. Will create a PR shortly.

[marchaos]

Merged this change.