add --cookies argument to read cookies from file

Hi,

Thanks for writing such a handy script, I didn't know about pyobjc until I saw webkit2png, and it's very cool! I added a command line switch to pass in a cookie file, which is parsed with cookielib and urllib2.

As an example, I use this Chrome Extension to copy out my cookies:
https://chrome.google.com/webstore/detail/lopabhfecdfhgogdbojmaicoicjekelh

And then I can use the cookies right off my pasteboard:

    ./webkit2png --cookies <(pbpaste) https://github.com/