Post processing of extraction result and adding new urls to the queue

[pbvahlst]

Add optional afterExtractor: function(data, options) which can be used for modifying data after successful extraction.

Furthermore it can be used for adding new urls to the extraction queue.

Should return one of undefined, null, [], {} or an object which should be saved:

The options object has the structure
{urls: []} where you can new urls which should be added to the queue-

If an object is returned test if data lives up the requirements of saving the data.
If new urls add them to extraction queue. The extraction queue should probably be
a Set instead of an array to avoid duplicates