src/day.c: Add day_write_stdout()

This function allows for writing stored items to stdout in a fashion
similar to day_write_pad(). This will be used as a convenience wrapper
when switching to day_store_items() in non-interactive mode and also
allows for easily implementing a "pipe-day" feature.

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

src/calcurse.h

@@ -668,6 +668,8 @@ int day_store_items(long, unsigned *, unsigned *, regex_t *);
 struct day_items_nb *day_process_storage(struct date *, unsigned,
                                          struct day_items_nb *);
 void day_write_pad(long, int, int, int);
+void day_write_stdout(long, const char *, const char *, const char *,
+                      const char *);
 void day_popup_item(void);
 int day_check_if_item(struct date);
 unsigned day_chk_busy_slices(struct date, int, int *);