Option to override default validator server

This feature is useful if you (like me) got blocked by w3 due to too many requests, or for whatever other reason choose to run a local instance of the validator service.

#### options.serverUrl
  Type: `String` <br/>
  Default value: `null`
 
 Supply a different validator server URL, for instance [if you run a local server](http://validator.w3.org/source/).
 Eg: `http://localhost/w3c-validator/check`