Add WriterT logger

[DennisVDB]

Here's an implementation of a WriterT logger (see #80). I based myself on how the current Writer logger is implemented.

I went through a handful of iterations and I'm still not sure if it's correct (it compiles). Especially, run: WriterT[F, G[LogMessage], ?] ~> F seems too constrained. I tried implementation it with the shape WriterT[F, G[LogMessage], ?] ~> H but couldn't.

[ChristopherDavenport]

As mentioned on gitter my holdout on this is that we either should put very large warning labels on it, or not include it. WriterT suffers that if any errors occur in F all values written are lost. Which makes sense for the abstraction but can be very surprising.

I'm inclined to the large warning label documentation, and then not include it in any supplied docs so that its available if people are certain thats what they want and go looking for it, but we sufficiently keep it away from wide use. It certainly has a place.

[DennisVDB]

The documentation I've added should straight to the point. What would you think about putting it under ...log4cats.extras.unsafe?

[ChristopherDavenport]

I think this is a fine location with the new warning.