Add configs for GeoJSON files (`.geojson`)

Ensure that clients interpret the content of `.geojson` files correctly
by serving them with the `application/vnd.geo+json` content-type.
http://www.iana.org/assignments/media-types/application/vnd.geo+json

Also, ensure that `.geojson` files aren't served with any of the `HTML`
document related headers, but are served compressed, and their specified
character encoding is UTF-8.

Ref: http://geojson.org/geojson-spec.html
Update `CHANGELOG.md`