This makes done tasks more stand out (like prioritized tasks) in the task list. Default highlighting is LIGHT_GREY, which seems to be modest and go well with the assumed dark terminal background.
Reintroduced the OSX-compatible sed filter that was introduced by commit 8b7e2e6aad and superseded by the PRI_X coloring generalization.
It allows you to assign a distinct color to any priority, not just A, B and C. Submitted to the todo.txt mailing list on 3-Jun-2010; committed on behalf of the author because there was no follow-up; cp. http://tech.groups.yahoo.com/group/todotxt/message/2619
Also committed this because it fixes a sed expression error when there are spaces in a $PRI_... variable, which occurred in my integration with Conky.
Note: This commit obsoletes and replaces the previous fix to colorization in commit 8b7e2e6aad.
Added tests for highlighting of priorities.
