Add --cache and --cache-filter arguments for caching.

[rottmann]

View Issue #75

Added 2 Arguments:
--cache [bytes] enables the cache, the optinal bytes are the size of the cache (default 10485760 10MB)
--cache-filter [regex] set a filter of which files should be cached (default \.(css|js)$ for .css an .js)

By default caching is disabled!

[coveralls]

Coverage decreased (-9.1%) to 85.156% when pulling 9a51661 on rottmann:master into 6186d23 on hexojs:master.