Add option for three sizes of placeholder images to ipsum parameter

Love schematic-ipsum for generating my test JSON.

Saw issue #3 and it seems like a good idea. I added it to the ipsum parameter, since it's a URL string.

Since you're going for randomized text to give the data a "lived in" feel, I used hhhhold.com because it delivers slightly randomized content, and not just the same static image to each instance like placehold.it would.