update to use cookies instead of headers

[artoonie]

It seems that nyt-s is now a cookie (and capitalized), not a header.

I don't know if this is universally rolled out, but the previous code doesn't work for my account, and this does. I assume this will work for everyone, but please do verify it still works for you.

Commit 1: The primary bugfix
Commit 2: Little changes to the plotly file to dynamically select a split year, to handle a 0 as the ceiling (which is what argparse provides on my machine I think), and to handle no data before the split year

[kesyog]

Thanks a lot for the find and the fix! It generally looks good to me, but I had some minor feedback.

[kesyog]

lgtm. thanks again!