[FRQ] - Allow extending Rollup config to add postcss

[sannajammeh]

Bunchee appears to be the only bundler out there for building server/client component libraries with actual code splitting. Great work!

We propose allowing for the user to extend the current rollup config to add support for postcss and similar. This will make the construction of these libraries much easier to work with.

It could be either:

1. Allow the user to have a postcss.config.cjs in the root to enable a postcss step
2. Allow the user to fully extend the rollup configuration in order to set up custom compilation and bundling for css in Rollup