Allow manual rule definition or add a map

[icedew0912]

While the grammar format is quite flexible, but Stacks alone are not enough.
For example, imagine a grammar like:

name = _{  ASCII_ALPHA * }
literal = {(name ~ already_number)| (name ~ not_prepared_number)  }

not_prepared_number= { ('0'..'9')+ }
already_number = ******* //  Need to look up the table or customize

Dealing with this externally is cumbersome and ungraceful.It would be nice if the custom section was allowed