Disable marker overlay by default

[mathiasbynens]

On pages like https://tibiamaps.io/blog/summer-update-2021 the performance impact of loading all markers by default is very obvious. Just like the exiva layer, we should disable it by default.

[mathiasbynens]

Ideally we could configure this via another data-* attribute e.g. data-show-markers="true".

@tiagomartines11, is this something you wanna take on? :)