[Performance] Cache minified files between restart in production

[FGRibreau]

Could it be possible to add a caching feature to Piler in order to not minify again files that didn't changed (maybe using sha1 to check for changes) between server restart in production.

I've got a nodejs server in production that takes more than two minutes to minify all the files when it starts, caching files will be a huge performance boost for piler and should contribute to its wide usage inside the nodejs community.

[pocesar]

implemented in branch 0.6.0, should be released soon