docs: function requireLogin (options) {
  	// merge with default options
  	options = mergeOptions({
  		'login': '/login',
  		'home': '/',
  		'user': 'user',
  		'referer': 'referer',
  		'excepts': []
  	}, options)
  	// add login to except page
  	options.excepts.push(options.login)
  
  	return function(req, res, next) {
  		var url = req.originalUrl
  		  , user = req.session[options.user]
  		// go to homepage where:
  		// 1. logged-in, and
  		// 2. on login page
  		if (user && url === options.login)
  	    	return res.redirect(options.home)
  	    
  	    // stay on page wherether:
  	  	// 1. logged-in, or
  	  	// 2. on login page, or
  	  	// 3. on non-required login page
  	  	if (user || options.excepts.indexOf(url) > -1)
  	    	return next()
  
  	  	// save referer into session
  	  	req.session[options.referer] = url
  	  	// redirect to login page if not login
  	  	return res.redirect(options.login)
  	}
  }