Add custom date label to `TimelinePoint`

[mathewjordan]

In some cases an author may want to add a custom label to a timeline point. So instead of outputting the granular date, allow an override of the datetime.

Ex: 1770 --> 1769-1773

In this case the date of 1770 is the point marker on the timeline while the dateLabel of 1769-1773 is what is rendered in the point label/description.

<TimelinePoint
  date="1770"
  dateLabel="1769-1773"
  title="Great Bengal Famine"
  summary="Starvation ravaged Company-controlled Bengal."
/>