Prevent an extra "../" in background-image url with cssPath filename option

if you specify a filename in options.cssPath, the path.relative function return an extra "../" level due to the filename.
Add a test if contains "." return cssPath dirname
Yeah - you're right - given paths aren't currently required to be terminated with a forwardslash. Unfortunately I didn't have the foresight to bake in that strictness from the start :/