feat(config): add two End dimension renders

feat: update central railways, remove decorative red lines

feat(stations): add Winter Garden

fix(config): rename second `nether` render to `nether_opposite`

docs(readme): update instructions

feat(config): add second nether render (opposite view)

feat(config): enable smooth lighting

feat(config): show render progress on the map itself

feat(config): use env vars instead of hardcoded directories

feat(config): specify path to the texture pack jar

fix(python): update code to Python 3

fix(config): fix an issue with importing markers

feat(config): add main config file that's directly used by Overviewer

style(markers_config): rename `overviewer_config` so it's less confusing

docs(license): update year to "2015-present"