[TW-67] per-field coloration rules

_David Patrick on 2010-02-26T22:23:06Z says:_

Choosing the right colors is extra hard when multiple rules can apply, but only one (or two) will take effect. The existing color directives can be triggered by any number of things, and I think we should consider extending the existing behavior with per-field directives.

for example

color.overdue=invert--due
color.due=bold--due
color.pri.any=bold--pri
color.pri.H=color220--pri
color.pri.M=color208--pri
color.pri.L=color112--pri
color.pri.none=on_color235
color.active=on_color22--active,description
color.tagged=yellow--tags
color.tag.bug=orange--id,tags

In this way fields can consistently use a set of colors, without being constantly overwritten by other rules. Directives will be able to highlight any part of the record.
At the same time, existing per-line directives will continue to be useful.
There is probably a better separator than "--" used in the example.. ":"?

Per-field coloration will have striking visual impact (no, it won't poke you in the eye) on differing sort keys, especially in interactive mode, emphasizing the sorted field.