Support overriding welcome_screen?

Currently, navigating to a SockJS server just displays Welcome to SockJS!

Lines 4 to 8 in e2369c3

    
           welcome_screen(req, res) { 
        
             res.setHeader('Content-Type', 'text/plain; charset=UTF-8'); 
        
             res.writeHead(200); 
        
             res.end('Welcome to SockJS!\n'); 
        
           },

It could be useful to override this with a custom message, such as a redirect to whatever webapp the SockJS server is a server for.

	welcome_screen(req, res) {
	res.setHeader('Content-Type', 'text/plain; charset=UTF-8');
	res.writeHead(200);
	res.end('Welcome to SockJS!\n');
	},