ENH: Allow listpri filtering with priority ranges.

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).

tests/t1250-listpri.sh

@@ -61,6 +61,42 @@ TODO: 0 of 5 tasks shown
 TODO: 0 of 5 tasks shown
 EOF
 
+cat > todo.txt <<EOF
+(B) smell the uppercase Roses +flowers @outside
+(X) clean the house from A-Z
+(C) notice the sunflowers
+(X) listen to music
+buy more records from artists A-Z
+EOF
+test_todo_session 'listpri filtering priority ranges' <<EOF
+>>> todo.sh -p listpri a-c
+1 (B) smell the uppercase Roses +flowers @outside
+3 (C) notice the sunflowers
+--
+TODO: 2 of 5 tasks shown
+
+>>> todo.sh -p listpri c-Z
+3 (C) notice the sunflowers
+2 (X) clean the house from A-Z
+4 (X) listen to music
+--
+TODO: 3 of 5 tasks shown
+
+>>> todo.sh -p listpri A-
+2 (X) clean the house from A-Z
+--
+TODO: 1 of 5 tasks shown
+
+>>> todo.sh -p listpri A-C A-Z
+--
+TODO: 0 of 5 tasks shown
+
+>>> todo.sh -p listpri X A-Z
+2 (X) clean the house from A-Z
+--
+TODO: 1 of 5 tasks shown
+EOF
+
 cat > todo.txt <<EOF
 (B) ccc xxx this line should be third.
 ccc xxx this line should be third.