Make TLS root CA loading more extensible

[shadowbq]

In config.go there is hardcoded papertrail CA loading

We should be able to point at a root CA file instead in the config or flag options

	// Make this more extensible
	// add the papertrail root CA if necessary
	if c.Destination.Protocol == "tls" && c.Destination.Host == "logs.papertrailapp.com" {
		c.RootCAs = papertrail.RootCA()
	}

[shadowbq]

More reasons why this becomes a bad idea.. vedarthk@e87f17e