Add --cookies option to pass in a cookie file

*This is a re-pullrequest of #31, but against the most recent master (since I'm a bit more up to speed with git now)*

Use a browser extension such as:

https://chrome.google.com/webstore/detail/cookietxt-export/lopabhfecdfhgogdbojmaicoicjekelh

...to dump cookies into a file, and then pass in the filename with
"--cookies FILENAME". You can also copy the cookie file content, and
then do "--cookies <(pbpaste)" to skip the intermediate file.