Support defining custom npm registry in rfbrowser init

[aaltat]

If rfbrowser init is run in environment where public npm register is not available supporting npm install —registry could be handy.

[fhennig42]

I think you can just use the npm_config_registry environment variable like this:

npm_config_registry=https://registry.example.org rfbrowser init

[aaltat]

This is about same as #2135 Closing this one as duplicate.