File system cache in production

[hlship]

It would be very nice to be able to use the file system cache option in production ... so that, for example, the server could be back up and running after a crash, quickly.

As noted in the existing documentation, however, the contents of the cache are not just a product of the input source files, but also other runtime configuration: exact versions of libraries, plus potentially other settings (such as Jade settings, see #16).

Perhaps the application could provide a version number, or build timestamp, or something equivalent, that could be incorporated into the path to the cache folder; something that would be the application's responsibility to change when the configuration changed.

Interestingly, even if the cache was wiped out but rebuilt to the same content, the asset checksums would be the same, so many browsers would already have resource content locally.