feat: pitchfork extractor (#439)

* generate the custom extractor and get the first test to pass

* add the basic extractors (title, author, date, etc)

* select the score as well as the review text, and break the content test

* prepend the score to the content

* get the date from the datetime attribute

* mangle this test a little, but just a little (it does work properly)

* move from prepending the score to the review text to adding it as a custom field in the extractor

Author: mgeraci