Implement caching for on-demand compiling with plessc

Implemented caching feature (that exists in main module) to `plessc` command-line tool.
Added `-q` key to `plessc` in order to enable this new feature.

The code if based on this example from `lessphp` documentation: 
http://leafo.net/lessphp/docs/#compiling_automatically

Generates `.lessphp` files beside corresponded less-files compiled, and then tries to read them and use as cache. 
Rewrites cache files when needed (`$cache_new["updated"] > $cache["updated"]`).