Support for persisting cookies

[nitrocode]

I'd like to analyze the network traffic if I can persist my cookie generated from a previous run.

So for instance with curl I can

# save a cookie
curl -c mycookie https://someurl
# reuse a cookie
curl -b mycookie https://someotherurl

And in confess.js I'd like to either create one using the same switch or use one

# save a cookie
phantomjs confess.js -c mycookie https://someurl
# reuse a cookie
phantomjs confess.js -b mycookie https://someotherurl

If I can figure it out, I'll try to make a PR