Don't hang on to the first Logger instance you see

If the main app instance hangs on to the logger that ::Middleman::Logger.singleton returns, then subsequent calls to re-init the logger won't have any effect (for instance, when setting up the preview server's logger based on CLI params).

Redefining logger to be a pass-through to ::Middleman::Logger.singleton instead of an ivar seems more in keeping with the sprit of a singleton, anyways.

This fixes an issue where running `middleman server --verbose` doesn't output any debug info.