[mandelbrot.py] Use argparse to simplify command line parameter parsing.

The produces very similar output as before. The code size could be reduced further by removing the "alias" variable section:

    (xL, yU, xU, yL) = options.coord
    pixelWidth = options.width
    startColor = options.color
    colorRange = options.colors
    outputName = options.output
    maxDepth = options.depth

and just using the option.* name everywhere in the code (ie, use `options.width` instead of `pixelWidth`).

-David