simulate() support for custom events on shallow rendered children

Any thoughts on having simulate run arbitrary custom events on a shallowly rendered child? Ie ideally I'd like to be able to call simulate on any of a shallow rendered childs props that call a function.

ex calling `simulate('customEvent')` on ` would run`func()` on a shallow rendered component.

This would allow us to assert functions that might not run on standard dom events without having to write a full integration test.

We could create a new function shallowSimulate() or something if we're worried about the interactions on fully `mount()`ed components.