Add a way to get a partial message from a URL

[4Kaylum]

commands.MessageConverter essentially does this already - what's the difference if I add a way to do it in the client instance? My issue will be with getting state, but I suppose I can do that by making this a method of Client and going from there.

[4Kaylum]

This doesn't work very easily for threads. Should we ignore that threads exist? I don't think we can.