Implement Sticky Charts and Sticky Canvas

[KarthikRIyer]

It'd be great to have sticky charts, in other words a sticky chart is a panel that sticks to the center of the screen as a background once it reaches the center of the screen upon scrolling. The chart can be the size of a single panel, or it may also be like the canvas .

Simple, sticky chart example: https://google.github.io/scrollytell/examples/sticky_chart.html
Fancy panels with sticky fullsize canvas example: https://google.github.io/scrollytell/examples/segmented_sticky_fullsize.html

[am15h]

Added center docked sticky chart.