Add `follow` option parameter

When using LeafletSlider to scan through a very large dataset that had overlapping paths, it became hard to determine what was going on at the end of the timescale. In order to better display such data, I found it to be useful to be able to remove the Markers off the map in a programmatic way. The method I came up with was to specify the number of Markers that I wanted to be displayed on the screen at any one time. The `follow` parameter is how to tell LeafletSlider to act in this way. For displaying data that forms a path, this meant that the display would create sort of a moving 'caterpillar' of data.