So far, the listpri action only supports a single priority. Allowing priority ranges (e.g. todo.sh listpri A-C @work) is a simple but useful enhancement.
Note: The syntax extension only clashes with the [TERM] filtering in a few corner cases, and this can be worked around (e.g. "todo.sh listpri A-Z A-Z" lists all prioritized tasks containing the text A-Z).
The regexp for the priority wasn't anchored to the beginning of the task. (As the filtering is done inside the _list pipeline, the task number has already been prepended.)
Also, by passing the regexp directly to _list, a case-insensitive search was performed, so despite [A-Z], lowercase characters were picked up, too. Need to make use of post_filter_command to inject a separate, case-sensitive grep into the pipeline.
Bonus: Added test for highlighting of listpri command.
