Make day_store_items() public

Remove the "static" keyword from day_store_items(), so that it is
accessible from other compilation units. Also, allow for discarding the
event/appointment counters by passing NULL pointers and move the
"regex.h" header inclusion to "calcurse.h".

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

src/args.c

@@ -41,7 +41,6 @@
 #include <limits.h>
 #include <getopt.h>
 #include <time.h>
-#include <regex.h>
 
 #include "calcurse.h"