Allow building a Logger yourself

[KodrAus]

Previously, I removed the ability to build and get a hold of a Logger type, because it relies on thread-local state that was configured using the style options of the first Logger to write a record.

This PR fixes up that situation so multiple Loggers can log on the same thread using the styles they were originally configured with. Each time we access the thread-local buffer we check to see if its configuration matches the current Logger. If it doesn't then we drop it and create a new one.

[KodrAus]

I've also added an example of using a Logger to write records directly, without using the log macros. It also works as a smoke test for the changes in this PR.