Export JS chunk -> CSS chunk dependency information

[tivac]

Feature Use Case

https://twitter.com/tivac/status/1085567955092496384

The rollup plugin already collects all this information (assuming #539 gets fixed), so making it available as JSON would allow for extra rollup post-processing steps that do all sorts of fun things.

Feature Proposal

Add a new option to the rollup plugin like meta that is either truthy or a filepath. The rollup plugin can then output a JSON file at that location that has at least one key in it, dependencies.