Add support for caching different --flags

[mixu]

Right now, caching is quite conservative: it will rebuild if any of the --flags change. However, a common use is to do two kinds of builds: 1) dev builds (no minification) and 2) production builds (minification).

I think it would be good to support caching with different options transparently so that if you alternate between a dev build and a production build, both will get cache hits.

[mixu]

Implemented in v2.1