Priority rule handling for colour overrides.

Making this issue partly as an explicit record of a personal docs todo (I will try to get to updating the docs soon) and partly to ask a question about changing behaviour.

Themeing colour overrides need to be parsed after colour includes. This may not be obvious based solely on the docs. If you have:
```
color.burndown.done=on rgb010
color.burndown.pending=on color9
color.burndown.started=on color11

include no-color.theme
```

You will have no colours. If you have:
```
include no-color.theme

color.burndown.done=on rgb010
color.burndown.pending=on color9
color.burndown.started=on color11
```
You get your colours.

This should probably be made more apparent, even if it is a common behaviour many configuration parsers follow.

The open question: should we implement some kind of priority rule that prioritizes `taskrc` over theme files, such that you could put those lines in any order within taskrc? This seems common sense to me but I wanted to pose it as a question, in case there are good reasons to avoid this.

Review the taskrc and no-color.theme ordering examples, then trace how colour includes and overrides are currently parsed. Determine whether the intended outcome is clearer documentation, a priority rule, or both; done means the chosen behavior is documented and any agreed behavior change is verified.

Written by the indexing model from the issue text.