Allow passing in cowboy_protocol:opts()

 - Create 'start_listener/4' that accepts ProtoOpts for cowboy
 - Include default 'env' that is currently being set

My motivation is that I want to be able to set the cowboy_protocol option: `max_request_line_length`.

Question 1: Does this fit with leptus design?
Question 2: Does this require changes to leptus:upgrade to ensure the ProtoOpts persist past an upgrade?