Add an option to choose the default layout that will be used when adding

a new tag dynamically. Feature #71 requested by myself ^^

src/structs.h

@@ -348,6 +348,7 @@ typedef struct
      Bool tagautohide;
      Bool tagnamecount;
      char * tagdefaultname;
+     char * tagdefaultlayout;
      uint pad;
      int status_timing;
      char *status_path;