Support the 'mkdocs' logger name, in case plugins refer to it directly

Make it a sub-logger of the 'properdocs' logger.

Author: oprypin

properdocs/__main__.py

@@ -96,11 +96,17 @@ def format(self, record):
 class State:
     """Maintain logging level."""
 
-    def __init__(self, log_name='properdocs', level=logging.INFO):
-        self.logger = logging.getLogger(log_name)
-        self.logger.setLevel(level)
+    def __init__(self):
+        self.logger = logging.getLogger('properdocs')
+        self.logger.setLevel(logging.INFO)
         self.logger.propagate = False
 
+        # Also support legacy logger name in case plugins refer to it directly.
+        mkdocs_logger = logging.getLogger('mkdocs')
+        mkdocs_logger.parent = self.logger
+        mkdocs_logger.setLevel(logging.NOTSET)
+        mkdocs_logger.propagate = True
+
         self.stream = logging.StreamHandler()
         self.stream.setFormatter(ColorFormatter())
         self.stream.name = 'ProperDocsStreamHandler'