Load is08601-js-period lib in heatmap_with_time plugin (#1228)

Author: Conengmo

folium/plugins/heat_map_withtime.py

@@ -161,6 +161,10 @@ def render(self, **kwargs):
         assert isinstance(figure, Figure), ('You cannot render this Element '
                                             'if it is not in a Figure.')
 
+        figure.header.add_child(
+            JavascriptLink('https://rawcdn.githack.com/nezasa/iso8601-js-period/master/iso8601.min.js'),  # noqa
+            name='iso8601')
+
         figure.header.add_child(
             JavascriptLink('https://rawcdn.githack.com/socib/Leaflet.TimeDimension/master/dist/leaflet.timedimension.min.js'),  # noqa
             name='leaflet.timedimension.min.js')