add remove_from_default boolean option to route_to_stream function

[kroepke]

this allows remove messages from the default stream like the stream router already does

with the remove_from_stream function any arbitrary stream can be removed. it always makes sure that the message is at least on the default stream. dropping messages this way is not supported, the user must use drop_message for that purpose.

fixes #117

[joschi]

@kroepke Please adapt the functions documentation in https://github.com/Graylog2/documentation/ accordingly.