add "write_to" parameter to wml_message

"write_to" lets the wmldev specify to where the message should be
written. Defaults to the previous behaviour.

The intention is for example that one might want to give a summary  as a
chat message and a very detailed information in the stderr file.
replace write_to with to_chat in [wml_message]

as shadowm suggested, because we are never in the situation that we want
this information in the chat but not in the stderr.