Marker overlay: add ability to select marker pack

[mathiasbynens]

Currently the marker overlay shows the default marker pack. It would be nice to select other packs (but not necessarily all of them). The most important other pack right now would be the PoI pack — it would be super useful to visualize which PoI areas we’ve already marked without having to actually install the pack locally. cc @tiagomartines11

[mathiasbynens]

I’m thinking of letting the embedder handle this via a data-marker-pack="minimap-with-grid-overlay-and-poi-markers/markers.json" attribute on the <div id="map"> element.

[mathiasbynens]

The PoI pack can now be viewed at https://tibiamaps.io/map/poi.