add the ability to stop the crawl

I had the need to stop the crawl at will, and so I added this new feature.

There's a new method that client code can call from within a page block, and will make the crawl stop after the current page is processed. The crawl is stopped nicely, not by forcefully killing the tentacle threads, but by clearing the queues so that the natural ending process occurs.

Hope this helps someone out there.
@gnapse why u clear the queue? What if u want to restart the processing? Would u submit this PR in the fork called [Medusa](https://github.com/brutuscat/medusa)?