Feature: add 'extra' attribute to parsed output

This is for #7. Had to update a lot of the tests because checking in the 'assert' listener doesn't include the extra lines (since they haven't been parsed at that point). 

I don't know if you would want to defer the 'assert' events for all of their 'extra' lines to be parsed, that's not in this patch but I could refactor to add that if desired.