Override settings from the command line

Add a --setting-overrides KEY=VAL command line option to override
default settings or those defined in settings files. This adds
flexibility in running Pelican and helps reduce sprawl of settings
files. Cast int and str setting overrides to their respective types.
Support other setting types by treating them as JSON. Fall back to JSON
when an override typecast errors. This should make it possible to set
int values to None, resp. to JSON 'none'

pelican/settings.py

@@ -1,6 +1,7 @@
 import copy
 import importlib.util
 import inspect
+import json
 import locale
 import logging
 import os
@@ -658,3 +659,23 @@ def configure_settings(settings):
             continue            # setting not specified, nothing to do
 
     return settings
+
+
+def coerce_overrides(overrides):
+    coerced = {}
+    types_to_cast = {int, str}
+    for k, v in overrides.items():
+        if k not in overrides:
+            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