Add more ENV variables

Hi:

Please, add the possibility that flags web.telemetry-path and web.listen-address could be fetched from an env variables too and not only from flags.

BTW, include in the documentation the default values for web.telemetry-path and web.listen-address

Change the default values for web.listen-address FROM :9187 to 0.0.0.0:9187

listenAddress = kingpin.Flag("web.listen-address", "Address to listen on for web interface and telemetry.").Default(":9187").String()

Regards

Thanks so much