log: Support -vvvv to not filter logs at all

docs/changelog.rst

@@ -26,6 +26,10 @@ v0.20.0 (UNRELEASED)
 - Make the ``mopidy`` command print a friendly error message if the
   :mod:`gobject` Python module cannot be imported. (Fixes: :issue:`836`)
 
+- Add support for repeating the :cmdoption:`-v <mopidy -v>` argument four times
+  to set the log level for all loggers to the lowest possible value, including
+  log records at levels lover than ``DEBUG`` too.
+
 **Configuration**
 
 - Add support for the log level value ``all`` to the loglevels configurations.

mopidy/utils/log.py

@@ -12,6 +12,7 @@ LOG_LEVELS = {
     1: dict(root=logging.WARNING, mopidy=logging.DEBUG),
     2: dict(root=logging.INFO, mopidy=logging.DEBUG),
     3: dict(root=logging.DEBUG, mopidy=logging.DEBUG),
+    4: dict(root=logging.NOTSET, mopidy=logging.NOTSET),
 }
 
 # Custom log level which has even lower priority than DEBUG