Use readFileSync and writeFileSync for better performance

Asynchronous functions have very unreliable performance, taking from 1ms up to 600ms.
Synchronous fs functions give much better and predictable performance (~1ms).

Related issue: https://github.com/gulpjs/gulp/issues/568

PS Not sure if you're ready to accept it, but this is currently a pressing issue for me (makes Gulp unusable). I'm assuming `vinyl-fs` is used for the build tools and runs on desktop, so there is no penalty for sync calls. This could be made configurable though.