Support Enviorment variables in config file

[josepowera]

If config file supports reading values from enviorment variables, it greatly simplifies option of running RedisShake as Kubernetes Job (you can specify target.address for example). https://github.com/pelletier/go-toml/issues/797

Similar to https://docs.influxdata.com/telegraf/v1.21/administration/configuration/#set-environment-variables