Allowing specifying box_alphas, box_widths and other features for individual layout items

[lolipopshock]

Candidates:

1. lp.draw_box(image, layout, box_alpha=[0.1, 0.2, ...])
2. lp.draw_box(image, layout, box_alpha_based_on_scores=True)

Also we could change the coloring configs in a way similar to Matplotlib:

1. lp.draw_box(image, layout, box_colors=[...])

[lolipopshock]

Setting box_alpha_based_on_scores is too specific; not implemented in #129