ENH: Print usage help for custom action.

Currently, the only way to get usage help for a custom action is to use "todo.sh help" and scroll / search for the action name. (Or try to call the action without / with invalid parameters to hopefully get a one-line syntax summary.)
This extends the help command to print the usage of optionally passed custom action names.

tests/t8020-actions-help.sh

@@ -0,0 +1,71 @@
+#!/bin/bash
+
+test_description='actions help functionality
+
+This test checks listing the usage help of a custom action.
+'
+. ./test-lib.sh
+
+unset TODO_ACTIONS_DIR
+test_todo_session 'custom action help with no custom action directory' <<'EOF'
+>>> todo.sh help foo
+TODO: No actions directory exists.
+=== 1
+EOF
+
+mkdir .todo.actions.d
+make_action()
+{
+	cat > ".todo.actions.d/$1" <<EOF
+#!/bin/bash
+[ "\$1" = "usage" ] && {
+    echo "    $1 ITEM#[, ITEM#, ...] [TERM...]"
+    echo "      This custom action does $1."
+    echo ""
+    exit
+}
+echo "custom action $1"
+EOF
+chmod +x ".todo.actions.d/$1"
+}
+
+make_action "foo"
+make_action "bar"
+make_action "quux"
+
+test_todo_session 'custom action help' <<'EOF'
+>>> todo.sh help foo
+    foo ITEM#[, ITEM#, ...] [TERM...]
+      This custom action does foo.
+\
+
+>>> todo.sh help bar
+    bar ITEM#[, ITEM#, ...] [TERM...]
+      This custom action does bar.
+\
+EOF
+
+test_todo_session 'multiple custom actions help' <<'EOF'
+>>> todo.sh help foo bar
+    foo ITEM#[, ITEM#, ...] [TERM...]
+      This custom action does foo.
+\
+    bar ITEM#[, ITEM#, ...] [TERM...]
+      This custom action does bar.
+\
+EOF
+
+test_todo_session 'nonexisting action help' <<'EOF'
+>>> todo.sh help doesnotexist
+TODO: No add-on action "doesnotexist" exists.
+=== 1
+
+>>> todo.sh help foo doesnotexist bar
+    foo ITEM#[, ITEM#, ...] [TERM...]
+      This custom action does foo.
+\
+TODO: No add-on action "doesnotexist" exists.
+=== 1
+EOF
+
+test_done