[#1] lasse_handler and related behavior implementation

Implemented cowboy handler for SSE.
You may want to pass the ``Req`` in this callback, too. And/or give the callback module a chance to tell you to just respond with an error instead of start looping. For instance, you can change the init callback to be something like:
```erlang
-callback init(InitArgs :: any()) -> {ok, State :: any()} | {error, StatusCode :: pos_integer(), ResponseBody :: binary()}.
```
…or…
```erlang
-callback init(InitArgs :: any(), Req :: cowboy_req:req()) -> {ok, NewReq :: cowboy_req:req(), State :: any()} | {error, NewReq :: cowboy_req:req()}. %% The response code and body should've been set on the callback module
```