Store frameworks in a shared cache

[Thomvis]

We build a lot of apps with shared dependencies and could greatly benefit from this project iff it would use a shared cache for storing and retrieving the compiled frameworks. I imagine it to be in a folder like ~/.cocoapods/prebuilt. The cache structure would involve subdirectories for xcode version, platform, spec version/hash to make sure you're getting the right binary.

Is this a use case that would fit with this project? If so, I'd be happy to see if I can start a PR for it.

[ivanmoskalev]

+1, would benefit greatly from this.