Feature Request: An option to limit the prebuilt package cache

[Gurkenglas]

Keeping prebuilt packages around takes hard drive space and saves on time that would be needed to rebuild them. Packages that will never again be used unnecessarily take up real estate. A package should be more readily thrown away if the ratio between the space it takes and the time it would take to rebuild is high. I wouldn't be surprised if some caching strategy manages 80% of the time savings with 20% of the space.

[mgsloan]

Considering that space is cheap, this isn't really a priority atm. Having a good way to remove things / GC them would indeed be great though - #133