feat(server): handle_QUIT callback

In case the implentation needs to clean up.

src/gen_nntp.erl

@@ -711,7 +711,16 @@ handle_command(<<"HELP">>, Client) ->
 % MUST acknowledge the QUIT command and then close the connection to
 % the client.
 handle_command(<<"QUIT">>, Client) ->
-  {stop, normal, <<"205 Connection closing">>, Client};
+  #client{module = Module, state = State} = Client
+  Client1 = case erlang:function_exported(Module, handle_QUIT, 1) of
+    false ->
+      Client;
+    true ->
+      {ok, NewState} = Module:handle_QUIT(State),
+      Client#client{state = NewState}
+  end,
+
+  {stop, normal, <<"205 Connection closing">>, Client1};
 
 % Any other commands.
 handle_command(Command, Client) ->