Added specialMarker parameter

[nagelp]

Thanks for this great little script! I've added an additional feature that allows to provide a "special" marker. I use it to visualize the geographic middle point of a bunch of coordinates (people who want to meet).

Try out the special marker functionality here: https://data.p173.de/midpoint/?center=51.49,0&zoom=8&marker=51.49,0&marker=51.3,0.2&specialMarker=51.523774,-0.158431