cache package downloads

in some usage cases it will happen to setup several similar wp sites and wp-cli will download the same packages several times (core, plugins, themes), it would be nice to cache them to speed up things (maybe enabling the feature only if cache location is manually set).

I see the plugins and themes downloads are handled by wp and I haven't checked how cumbersome would be to insert the archives caching logic there (maybe custom upgraders?). Core download are handled by wp-cli and it seem possible to insert the cache logic in the core command.

How does it sound?