add categories `colors` to display.metadata.attr

[hneutr]

it'd be cool to apply your own colorschemes to things that aren't scalar.

Formats:

{
    'attrName' : {
         categories: [ "cat1", ... ],
         colors: [ "cat1color", etc ],
    },

{
    'attrName' : {
         colors: {
               "cat1" : "cat1color",
              ....
         },
    },

would work for binary data too. Binary inverting would be interesting, though. (probably still allow it?)