Add struct parser for generic add-on commands.

src/partiview.H

@@ -91,6 +91,14 @@ struct _ppui {
   Fl_Plot*  hrdiag;
 };
 
+struct parser {
+  int (*parsefunc)( struct stuff **stp, int argc, char *argv[], void *etc );
+  char *whose;			// name
+  void *etc;			// client data
+  struct parser *next;
+};
+
+extern struct parser *parsers;
 
 extern struct _ppui  ppui;