PX color by timedelta

[harahu]

When you domain is time series data, you sooner or later have to plot data of the Timedelta type. It would be nice to be able to understand what I am looking at when plotting, and coloring by, such data, which is hard when it is interpreted as an int in both hover text and color bar tick text.

Similar to #2450, but covering the timedelta data type, rather than datetime.

See also #799 and #801 for similar requests for the x and y axes.