Force root logger to be of FatalLogger class

This enforces FatalLogger policy upon pelican plugins
that might contain codes such as:

    import logging
    log = logging.getLogger()

    log.warning(...)
    # or
    logging.warning(...)

pelican/log.py

@@ -165,6 +165,8 @@ class FatalLogger(LimitLogger):
 
 
 logging.setLoggerClass(FatalLogger)
+# force root logger to be of our preferred class
+logging.getLogger().__class__ = FatalLogger
 
 
 def supports_color():