Add introduction to the new online help system

This adds a very basic explanation of the new help command that can be
displayed using ":help intro". This introduction is also added as
default page so that users can simply type ":help" instead.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/calcurse.c

@@ -503,6 +503,9 @@ static inline void key_generic_cmd(void)
 		char *topic = strtok(NULL, " ");
 		char path[BUFSIZ];
 
+		if (!topic)
+			topic = "intro";
+
 		snprintf(path, BUFSIZ, DOCDIR "/%s.txt", topic);
 		if (io_file_exist(path)) {
 			wins_launch_external(path, conf.pager);