improve W005 checker by properly parsing URL and checking filename

CC: @hnrch02 
Hmm, but then you can run into `use strict` issues.
JSHint complains about "global" strict (http://www.jshint.com/docs/options/#strict ),
and putting `use strict` in every top-level function is annoying.
Hence, the dummy IIFE that's `use strict`-ed.
I'm all ears if you have a good alternative though :smile: