Export log formatting

[ygale]

Make the advice in the makeLogger haddock practical:

For logging during app startup, it recommends creating a Logger, using during startup, and then supplying it to makeLogger when creating the Yesod instance. But for that to be practical, you need access to the default implementations of the various logging-related methods of Yesod and/or the formatLogMessage function. Unfortunately, none of those are available before the Yesod instance is actually created, leading to a chicken-and-egg problem.

This PR moves the default implementations of the logging-related methods to separate functions that don't depend on the Yesod instance, and exports all of those plus formatLogMessage from Yesod.Core.

[gregwebs]

Great!

[snoyberg]

Awesome, thanks!