assets handler: automatically configure `Cache-Control` on startup

[jknack]

{
  assets("/assets/**");  
}

Asset handler will be automatically configured with assets.cache.maxAge property on startup. This property control the Cache-Control header, default value is -1 (no cache).

Allowed values includes

• zero/negative number: turn off the cache control header
• positive number: represents the number of seconds the set in the Cache-Control header
• duration string: like 1d, 30s, 60m, 365d, etc...