Add setup callback

[joefiorini]

This will allow Grunt users to further configure their applications beyond
just middleware, including mounting applications at specific endpoints.

[joefiorini]

According to @sidwood (thanks) on #16, I can do this same thing in the middleware callback by doing:

middleware: function(connect, options){
  return [
    connect().use("/styles/styles.css", sass.serve(options.base + "/../styles/styles.scss")),
    connect.static(options.base),
    connect.directory(options.base)
  ];
},

Now that I've seen it, it makes sense, but I did not realize that connect().use(...) returns middleware. I'm not sure if this plugin should depend on users having that kind of in-depth knowledge of connect. I think this PR makes it much clearer what I need to do to access the functionality I need. If we don't want to merge this, then maybe I could at least update the README to show an example of this?

[sindresorhus]

Ya, would accept a readme PR ;)