Add color coding to show substitutions

[NicMcPhee]

Google Sheets (and presumably other spreadsheet tools) provide nice color coding to make it easier to see which cells are being referred to in a formula. We could do something similar, where variables in the rule could be colored, and the matching (sub)expressions in the first and second expressions could be colored the same way.

We'd have to extract info from the matching code on the actual substitutions that are being done, but I don't think that will hard.

It would be nice to try to use a large-ish color-blind-friendly palette of colors to maximize the usefulness of the coloring. We might want a checkbox people can use to turn this feature on and off.