Support option for custom cdn #133
Description
Expected behaviour
Have the option to specify a custom cdn to download the Highcharts library from.
It could be asked for through prompt or set as an environment variable. Basically the same options as how the version can be specified.
Actual behaviour
cdnUrl is hard coded resulting in Highcharts is always downloaded from https://code.highcharts.com
Reproduction steps
Run npm install or node build.js.