Add direct support for Splat/ReactiveUI initialization

[ryanwersal]

Currently swapping out the Locator, without doing further per-instance work, results in breaking RxApp and Splat's expectations about what services are available and what type they are (since they'll get auto mocked).

It would be ideal to make this as much of a one liner as possible since Splat is used extensively with ReactiveUI so it doesn't seem outlandish to expect it (though offering the option of not requiring ReactiveUI may sink this idea).