Limit and filter logs

Drop duplicates logs.
Allow for logs to be grouped, enforcing a maximum number of logs per group.
Add the LOG_FILTER setting to ask from the configuration file to ignore some
logs (of level up to warning).

pelican/tests/test_pelican.py

@@ -83,7 +83,7 @@ class TestPelican(LoggedTestCase):
         mute(True)(pelican.run)()
         self.assertDirsEqual(self.temp_path, os.path.join(OUTPUT_PATH, 'basic'))
         self.assertLogCountEqual(
-            count=4,
+            count=3,
             msg="Unable to find.*skipping url replacement",
             level=logging.WARNING)