Support date-time binning in histograms

When date-time *durations* are expressed with a numerical value, plotly's unit is the millisecond (ms).
This is different in R, where the `POSIXt` class uses seconds, and the `Date` class uses days.

This PR offers the corresponding conversions for the width (size) of (`x`) bins in histograms.

See these successful `ggplotly` conversions when dates are of class `POSIXt`:
https://plot.ly/~marianne2/336/deaths-by-police-across-time-by-region/,
when dates are of class `Date`:
https://plot.ly/~marianne2/358/deaths-by-police-across-time-by-region/

/cc @chriddyp @alexcjohnson @jackparmer

whitespace?