Logger: formatting messages

As you can see, the situation is still not ideal...  https://github.com/Kdyby/Monolog/commit/c56029498ff2f23773b22a5a8fd777ab8a6b4341, https://github.com/Kdyby/Monolog/commit/c4a6b4d256a3980631b31c85e4100451544ed368 

This will help a lot. Or do you have any better idea?
What about `$line = $this->formatLogLine(...)`? To express the difference between line and message.