Added file marks; fixes issue #94

- Command it_toggle_image_mark (bound to 'm') toggles mark of current
  image
- Command it_navigate_marked (bound to 'N'/'P') can be used to go to
  the next/previous marked image
- When option -o is given, all marked files get printed

types.h

@@ -69,6 +69,7 @@ typedef struct {
 	const char *path; /* always absolute */
 	const char *base;
 	bool loaded;
+	bool marked;
 } fileinfo_t;
 
 /* timeouts in milliseconds: */