make caching/watching part of core

[joeybaker]

The browserify tool chain is somewhat cumbersome because there's no easy way to access the internal cache of files. Watchify and browserify-incremental had to implement their own, and combining either with factor-bundle requires saving the bundle to the file system.

It would be ideal if paq offered an API to access it's internal cache of files + file contents in a persist-able way.

[ben-ng]

This is a good idea! The latency from node watch modules is really high and affects perceived performance.