Ability to add Images/Links to markers

[TConnor2003]

Feature Description: Add the ability to add images/links to dynmap markers.

• Additional context: This could be used to allow players either add images to allow better viewing of builds e.g. if the build is underground or with shaders enabled. or using the link they could link to a gallery or detailed description of the build. I have added an image below with a very basic example of what I mean

[JurgenKuyper]

marker descriptions are full html, with some sanity checks in between, thus, this should work imo