Allow a configurable env var prefix [#19]

I'd like to keep Prefix's scope fairly limited, because it has potential
to spiral into something magnificently complex. You get one prefix
setting, it goes in `settings` not `environment`, end of feature.

internal/helm/config.go

@@ -42,9 +42,17 @@ func (cfg *Config) Populate() error {
 		return err
 	}
 
+	prefix := cfg.Prefix
+
 	if err := envconfig.Process("", cfg); err != nil {
 		return err
 	}
 
+	if prefix != "" {
+		if err := envconfig.Process(cfg.Prefix, cfg); err != nil {
+			return err
+		}
+	}
+
 	return nil
 }