Added support to stop next middleware processing upon options.stopNext=true

Useful when same KOA website serves regular requests and also handles the pre-rendering interception and return right after the content is obtained from PhantomJS. Example: your Angular app hits your API KOA server. You also want to intercept the prerendering request in same stack and return right after. This allows you to not keep a stand alone stack just to mount koa-prerender alone.