When users specify any number of -q's on the command-line, we should not
show the source even if they have otherwwise configured Flake8 to do so.
Closes#245
Fundamentally on Flake8 2.x using -q altered the format of the errors
(and the behaviour a little) so it makes the most sense to implement
this logic with formatters rather than messy logic spread throughout
the project.
The FilenameOnly formatter will keep track of filenames already reported
and only print the name once while Nothing will print nothing.
Closes#180
