Add .extend to allow for extensions

Aphrodite has a `.extend` function [as seen here](https://github.com/Khan/aphrodite/#creating-extensions), which allows devs to extend Aphrodite with various features (right now it's only selectorHandlers).

It would be great to be able to do this in `react-with-styles` as well.

Start by reading Aphrodite's linked extension documentation and the existing selectorHandlers integration in this interface. Identify the entry point that exposes Aphrodite behavior, then verify that react-with-styles supports an equivalent .extend mechanism; the issue does not name specific files or tests.

Written by the indexing model from the issue text.