Implement io.Reader interface on msg

[wneessen]

I thought about using io.Copy for convenience reasons, such as dumping a message to log writer, but tbh this is not something I really need. I'm perfectly fine with current API =) However, as Msg now satisfies io.WriterTo it seems strange that you can't use it as an argument for widely used io.Copy. I think adding this makes sense.

Originally posted by @inliquid in #3 (reply in thread)