Accept filters for listcon, too.

This has been implemented for listproj already, for consistency in use and implementation, the same should be available for listcon, too.

tests/t1310-listcon.sh

@@ -55,6 +55,16 @@ test_todo_session 'listcon e-mail address test' <<EOF
 @con02
 EOF
 
+cat > todo.txt <<EOF
+(B) smell the uppercase Roses +roses @outside +shared
+(C) notice the sunflowers +sunflowers @garden +shared +landscape
+stop
+EOF
+test_todo_session 'listcon with project' <<EOF
+>>> todo.sh listcon +landscape
+@garden
+EOF
+
 cat > todo.txt <<EOF
 @con01 -- Some context 1 task
 EOF