Add more direct control of what samples have what colors

[fedarko]

This would allow users to still select general color schemes (e.g. tableau10), but also to modify individual samples' colors (like in Emperor).

This might be hairy to implement due to the same sorts of problems blocking #67 -- I'm not sure resolving this in an automatic fashion will be all that easy. Maybe later versions of Vega/Vega-Lite will make getting around these problems easier.

forum ref: https://forum.qiime2.org/t/boxplot-visualization-colours-through-qurro-and-songbird/13207