Add option to order by estimated prominence

main.hpp

@@ -69,6 +69,8 @@ extern std::vector<order_field> order_by;
 // not legal UTF-8, so can't appear as a real attribute name
 #define ORDER_BY_SIZE "\200size"
 extern bool order_by_size;
+#define ORDER_BY_PROMINENCE "\200prominence"
+extern bool order_by_prominence;
 
 int mkstemp_cloexec(char *name);
 FILE *fopen_oflag(const char *name, const char *mode, int oflag);