In the process of factoring out the field filtering, the "extra"
and "fullname" args are just passed in as a blank string now.
Extra was functionality that allowed a field modifier to be defined
as "filtername(arg1,arg2):field", and fullname was the name of the
field including any provided field modifiers. From grepping through
the add-ons on AnkiWeb, neither appears to have been used.
pytest will show what differs in simple assert statements
concurrent mode is supported with a plugin, but like nose2, concurrent
mode hides the cause of import errors, so I've left it off for now.
