New options similar to placehold.it service

My attempt to add optional height and background/foreground colors.

You can use following formats now:
http://placehold.local/80 (classic 80x80 image)
http://placehold.local/80/fff (80x80 image with white background)
http://placehold.local/80/fff/f00 (80x80 image with white background and red text)
http://placehold.local/80x20/fff (80x20 image with white background)