added ability to use different URL to access the Pirate Bay

Hello, I really liked this little program but in my country (and many others, I fear) `http://thepiratebay.se` is blocked by every ISP. To use the utility I implemented a `--url` or `-u` option that can be used to set a custom URL to access the website.

I went to the [piratebay proxies list](http://proxybay.info) and found out that http://pirateproxy.in actually works very well with kaizoku and I can access it.

Now I just put `alias kaizoku='kaizoku -u http://pirateproxy.in` in my shell dotfile and every time I launch kaizoku it just works, without polluting ~ with yet another configuration file. The default URL is still http://thepiratebay.se

I hope you like the addition, and very nice work :+1: