Don't bother validating class names if --no-css option is passed

The issues jorgebastida#27 and jorgebastida#37 are still around for
files called myfile_1.png and myfile_2.png :/

But instead of fixing that, I'd rather just tell glue not to generate
css. Turns out it will validate the css classes nonetheless. Bad, bad
glue. :bug:

This PR should fix that behavior.