Add --uppercase-hex

[WaffleLapkin]

--uppercase-hex makes dominant_colours output uppercase hex (#7DC07B), instead of lowercase (#7dc07b) when printing color codes.

I find uppercase hex to be easier to read, since all digits (including letters) are the same height.

I have some ideas on minor improvements/cleanups to the code, but I'm unsure if you are interested in that. LMK if you do!

[WaffleLapkin]

Hm. While playing around with dominant_colours I added a few more options, like --sort (sorts the output in "most common to least common" order, IMO should be the default), --amount (not sure if the best name, shows the % of the image that is closest to that colour, besacally the key for --sort), --seed (for the rng in kmeans), options to control resizing (max size, alg), --index-image (outputs an "indexed" image, using dominant colours), and --sort-index (kind of silly, sorts the output of --index-image, helpful to visually see how much of which color is there in the picture).

On one hand I do like being able play with all those options, but on the other hand I see how this kind-of clatters otherwise very simple CLI. What do you think @alexwlchan? Are you interested in any of this?