Parameters in the configuration block that start with _map_
are parsed using template toolkit and imported into the namespace of the
The prefix is stripped and the param is set to the result of the evaluation,
the value is interpreted as template and filled with the context:
_map_my_tt_param: my_prefix_[% context.my_context_key %]
If you just need a single context value, the dollar sign is a shortcut:
The values are accessible thru the $self->param call using the basename.