Inversion of control

[dwilt]

I'm a big fan of the concept of inversion of control with UI libraries because it allows the consumers to control the styling completely. Is there any reason it wasn't taken as an approach with this library? I think it would be helpful for a few reasons:

1. You'd be able to remove the as prop
2. You'd be able to remove documentation around having the users to include custom styling: https://reach.tech/styling#menubutton-as