[feature request] allow consumers to pass options to emotion

[MustafaHaddara]

import root from 'react-shadow/emotion is super useful, but for my use-case I'd like to be able to pass in a custom key to the createCache call here. I can imagine other consumers may want to override other emotion defaults as well.

I imagine this would also probably be relevant for emotion-ssr, styled-components, and material-ui, although I am less familiar with those.