Give more abstraction to the filesystem

[FranckFreiburger]

currently, sw-precache uses 'fs' (fs.statSync, fs.readFileSync, fs.realpathSync, fs.readFileSync, fs.readFile, fs.writeFile) to access files, whereas, in dev mode, webpack uses a memory filesystem.
In order to allow test sw-precache in dev mode, it could be conviennent to allow sw-precache to use this memory filesystem.

Proposed API:
add a new optional fs entry in params of generate(params, callback) ?