Allow user-defined context fields

assets/parser.d.erb

@@ -183,6 +183,8 @@ public struct <%= @grammar.prefix %>context_t
 
     /** User terminate code. */
     size_t user_terminate_code;
+
+<%= @grammar.context_user_code %>
 }
 
 /**************************************************************************

assets/parser.h.erb

@@ -172,6 +172,8 @@ typedef struct
 
     /** User terminate code. */
     size_t user_terminate_code;
+
+<%= @grammar.context_user_code %>
 } <%= @grammar.prefix %>context_t;
 
 /**************************************************************************