title: folium timeslider doesn't remove previous added marker in the map
body: <p>I am trying to plot markers using time slider. I have created timestamped geojson as follows:</p>
  
  <pre><code>def create_geojson_features(df):
  
      features = []
      for lat,lan,intensity,time in zip(df['latitude'],df['longitude'],df['intensity'],df['timestamp']): 
          time = str(time)
          feature = {
              'type': 'Feature',
              'geometry': {
                  'type':'Point', 
                  'coordinates':[lan,lat]
              },
              'properties': {
                  'time': time,
                  'style': {'Color' : color(intensity)},
                  'icon': 'Marker',
                  'color':color(intensity),
                  'iconstyle':{
                      'Color': color(intensity),
                      'fillOpacity': 0.8,
                      'stroke': 'true',
                      'radius': 7
                  }
              }
          }
          features.append(feature)
      return features
  features = create_geojson_features(df)
  </code></pre>
  
  <p>Here is my code to plot the markers</p>
  
  <pre><code>from folium.plugins import TimestampedGeoJson
  m = folium.Map([latmean,lonmean], zoom_start=11)
  
  TimestampedGeoJson(
          {'type': 'FeatureCollection',
          'features': features}
          , period='PT1H'
          , add_last_point=False
          , auto_play=False
          , loop=False
          , max_speed=1
          , loop_button=True
          , date_options='YYYY/MM/DD HH:mm:ss'
          , time_slider_drag_update=True
      ).add_to(m)
  </code></pre>
  
  <p>I have specified period as PT1H, as I scroll the slider the new points keep on adding. How can I remove old points and show new points only? Please help.</p>