Deep extended options must include `middleware` property

[amaxz]

Deep extended options must include middleware property, fixed code here:

  var config = enableMiddlewareShorthand(defaults, options, [
    'directoryListing',
    'livereload',
    'middleware'
  ]);

usage:

// options:
...
middleware: [
    serveStatic('../download', {
        index: false,
        setHeaders: function setHeaders (res, path) {
            res.setHeader('Content-Disposition', require('content-disposition')(path))
        },
        fallthrough: true
    })
],
...