Accept '0' as a level which doesn't suffix the shortname.

Some specifications don't actually have levels. "Fetch", for instance. And "html5".

This patch accepts `"level": 0` as a hint that the shortname shouldn't be suffixed. An alternative approach would be to make the `level` attribute optional, but since you'd explicitly made it required, I'm not sure you'd accept that patch. :)

WDYT?