Fix #2938

Get rid of the coerce_overrides() function.
Add the ParseOverrides argparse.Action to parse overrides.
Treat all extra settings values strictly as json values.
Test overrides.
Edit docs and cli help.

pelican/settings.py

@@ -1,7 +1,6 @@
 import copy
 import importlib.util
 import inspect
-import json
 import locale
 import logging
 import os
@@ -659,25 +658,3 @@ def configure_settings(settings):
             continue            # setting not specified, nothing to do
 
     return settings
-
-
-def coerce_overrides(overrides):
-    if overrides is None:
-        return {}
-    coerced = {}
-    types_to_cast = {int, str, bool}
-    for k, v in overrides.items():
-        if k not in DEFAULT_CONFIG:
-            logger.warning('Override for unknown setting %s, ignoring', k)
-            continue
-        setting_type = type(DEFAULT_CONFIG[k])
-        if setting_type not in types_to_cast:
-            coerced[k] = json.loads(v)
-        else:
-            try:
-                coerced[k] = setting_type(v)
-            except ValueError:
-                logger.debug('ValueError for %s override with %s, try to '
-                             'load as json', k, v)
-                coerced[k] = json.loads(v)
-    return coerced