Implement a Source cache to avoid over-eager emitting

[marcins]

This change implements a cache for Sources generated by this plugin so that on subsequent builds (i.e. incremental builds) we can re-use the same Source if the content hasn't changed.

This improves performance with Webpack's futureEmitAssets mode, where there is a cache based on Source referential identity that determines whether an asset should be re-emitted.

Fixes #7