New filter type: server_middleware

I added a new type of filter: **server_middleware**. The new filter type helps you customize server middlewares. For example:

``` js
hexo.extend.filter.register('server_middleware', function(app){
  app.use(function(req, res, next){
    res.setHeader('X-Powered-By', 'Hexo');
    next();
  });
});
```