Refactor distinguishable_colors API

I wanted to make this easier to call, and allow one to supply multiple seed colors.

CC @dcjones:
- I'm not at all confident I've provided the right defaults for l, c, and h. Your expertise is particularly desired here.
- If the user doesn't supply any seed colors, note the first color will be equivalent to LHCab(lchoices[1], cchoices[1], hchoices[1]). We should make sure that's a reasonable color.
- Is RGB the right default output type when one calls this simply with an integer argument, `distinguishable_colors(5)`? Or would a different type be better?