Adding a useStickyState hook, which saves state as cookie

[tumbleshack]

Question: should all the graphs share the same cookie for the showPastDays param?

I think not, since it would lead to an odd behavior whereby sliding one slider would result in another slider being in a different position upon component re-mount.

That said, I think it's worth considering because this would mean default behavior has users comparing apples-to-apples.

Will close #109

[xunhuang]

maybe one per graph type(/class)?

[tumbleshack]

Cool, that’s how it’s currently working. I’ll leave it as is

[xunhuang]

awesome! thank you.