Map markers

[lorenzschmid]

Adds simple configuration to add markers on the map, see also original MR form old repository under https://github.com/DigitaleGesellschaft/jekyll-conference/pull/15

Currently missing/todo:

• [ ] Add simple mean to exclude (avoid bloating sites not using markers)
• [ ] Add documentation to README
• [ ] Improve default configuration to not include marker on each site by default

[lorenzschmid]

Outdated.