add option to change x-axis to be "week of season" so that multiple seasons could be viewed overlaid

[nickreich]

Taking inspiration from the older flusightnetwork visualizations
, it would be nice to be able to overlay previous seasons on top of the "current season". Maybe this could be implemented not as a user-activated toggle, but just as a global option for the viz for a particular instantiation. In which case, perhaps there would need to be parameters about what weeks define the "season", so that previous data could be split based on that.