ENH: Allow concatenation of multiple priorities [-ranges] for listpri

It can be useful to filter for non-consecutive priority ranges. By enforcing uppercase in that added syntax, overlap with general TERM(s) (e.g. "foo-bar") can be mostly avoided (and in the rare case of having to filter by all-uppercase TERM(s) one can always pass the default A-Z filter, anyway).

tests/t1250-listpri.sh

@@ -96,6 +96,26 @@ TODO: 0 of 5 tasks shown
 --
 TODO: 1 of 5 tasks shown
 EOF
+test_todo_session 'listpri filtering concatenation of priorities and -ranges' <<EOF
+>>> todo.sh -p listpri CX
+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 ABR-Y
+1 (B) smell the uppercase Roses +flowers @outside
+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
+EOF
 
 cat > todo.txt <<EOF
 (B) ccc xxx this line should be third.