Support pre-processing of stitches css

[MrKou47]

Is your feature request related to a problem? Please describe. We are building a large mobile web app. For convenience, we are using px2rem to simply copy css generated from Sketch and it can transform px to rem automaticlly. However, in stitches I dont know how to implement this feature.

Describe the solution you'd like We can probably provide a global config like a function callback or a hook which can modify css property feature before generating css into style tag.