Add .extend to allow for extensions

[merlinstardust]

Aphrodite has a .extend function as seen here, 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.

[ljharb]

That's very aphrodite-specific; the goal of react-with-styles is to be interface-agnostic, which means all react-with-styles interfaces must have the same API.

If we add any feature - like .extend - to one interface, we'd have to be able to add it to all of them.

We could certainly make this package export a function that takes options that returns an interface - that might be one way to make it work.

[merlinstardust]

That's reasonable to me. And I think that's important to do as react-with-styles becomes popular, there will be many interfaces with interface-specific features.

[ljharb]

That's why I'm concerned about adding this feature - we need to be limited to the lowest common denominator of what all interfaces support. That's much more important than any individual interface feature.