fix vector map labels opacity

Fix vectorserverUrl not returning image

release v4.0.0

prefer faster oxipng compression

upgrade all dependencies

fix imagemin missing optipng binary

upgrade commonjs to nodejs modules

Add prod docker files

uningnore package-lock

Improve speed, upgrade deps, and minor fixes

Update README.md

Allow to customize header check fail message

Merge branch 'master' of github.com:jperelli/osm-static-maps

protect server with header check if available

Fix "error TS1046"