Extended isURL to also support probing for domains in the validated URL.

I find it useful, quite often, to validate an URL but at the same time check if any given domain is being used or not. Now it is possible to do this:

```javascript
var validator = require('validator'),
    options = {
		include: 'abird.co, stackoverflow.com, nodejs.org',
		exclude: 'google.com'
	};

validator.isURL('http://nodejs.org', options); // True - Matches at least one included
validator.isURL('http://powerball.com', options); // False - Does not match at least one included
validator.isURL('http://google.com', options); // False - Does not exclude all excluded
```

Let me know what you think.