Move HIDE_PRIORITY_SUBSTITUTION handling back

Ingo pointed out that this was where it was to prevent strings like (A)
from being replaced in the middle of a line.
Rename env vars from *_COLOR to COLOR_*

Per Ingo's request.  This provides consistency with COLOR_DONE.
Export COLOR_PROJECT and COLOR_CONTEXT

This way, they can be changed via the config file without the user
needing to do an export.
Suppress highlighting projects/contexts with -p

As pointed out by Ingo, projects and context highlighting was being
applied even if the user had set TODOTXT_PLAIN
No project/context coloring if -@ or -+ was used

Along the same lines as the previous TODOTXT_PLAIN fix, using -@ or -+
didn't work properly with context or project coloring enabled - the
color codes got inserted first, which made the pattern no longer match.
Add a bunch of project/context highlighting tests