Add stylis middleware support

[niksurff]

Should we add stylis middleware support via Babel plugin options that would be passed down to stylis.use?

[giuseppeg]

I was wondering if it'd make sense to let users define a function to transform the CSS that is passed to Stylis instead. Like:

const userTransform = state.file.opts.transform || (i) => i
transformedCss = transform(
    String(state.jsxId),
    userTransform(css)
)

This will in fact make styled-jsx extensible i.e. people could use SASS, I could drop styled-jsx-postcss etc.
Thoughts? cc @rauchg @nkzawa

[thysultan]

@giuseppeg BTW re: 171 stylis has nesting support out of the box.

[giuseppeg]

@thysultan yup those are awesome features, I think that if we add support for a single function people can apply Stylis on their own like they would do with anything else (eg. sass or postcss).
IMHO styled-jsx should only scope selectors which is something that I would like to discuss when we get close to v1.0.0.

[giuseppeg]

Fixed by #291