Add support for mixed rendering

[Saegrov]

Mixed rendering is a mode that works like shallow rendering, but where
you can specify a list of components you want to render even tho they
are deeply nested.

We have a use case where we are using shallow rendering and Cheerio for unit testing our Preact components. We are using a redux-connect like higher order component to connect some components to our global state.
When testing a component, that uses another component that is wrapped in a HOC, the shallow renderer stops at the HOC.

With mixed rendering we can white list components we want to render and test that the wrapped components are used correctly.

[Saegrov]

Had to do some commits to use this internally at our company. Closing this in favour of #50