adding support for simple convex polygons

@tmcw this is really just for fun -- I know that a few nodeschool lessons is not enough to get my code beyond the "abomination" phase. But I'd be grateful for any notes it elicits. In particular I don't know enough about geo to know what actually-useful randomized geojson looks like (which honestly means I probably shouldn't be writing scripts to generate it, but I tried to take inspiration from what was there).

No idea about the right way to handle the command line interface since I needed some extra params (which should perhaps be randomized instead -- see previous parenthetical).
Careful here - the `||` trick is great for non-numeric values, but it won't always mean what you think with numerics: http://mistakes.io/#479753b16f11ddd11ca3