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/tests/test_settings.py

@@ -7,8 +7,8 @@ from sys import platform
 
 from pelican.settings import (DEFAULT_CONFIG, DEFAULT_THEME,
                               _printf_s_to_format_field,
-                              configure_settings, handle_deprecated_settings,
-                              read_settings)
+                              coerce_overrides, configure_settings,
+                              handle_deprecated_settings, read_settings)
 from pelican.tests.support import unittest
 
 
@@ -304,3 +304,18 @@ class TestSettingsConfiguration(unittest.TestCase):
                          [(r'C\+\+', 'cpp')] +
                          self.settings['SLUG_REGEX_SUBSTITUTIONS'])
         self.assertNotIn('SLUG_SUBSTITUTIONS', settings)
+
+    def test_coerce_overrides(self):
+        overrides = coerce_overrides({
+            'ARTICLE_EXCLUDES': '["testexcl"]',
+            'READERS': '{"foo": "bar"}',
+            'STATIC_EXCLUDE_SOURCES': 'true',
+            'THEME_STATIC_DIR': 'theme',
+            })
+        expected = {
+            'ARTICLE_EXCLUDES': ["testexcl"],
+            'READERS': {"foo": "bar"},
+            'STATIC_EXCLUDE_SOURCES': True,
+            'THEME_STATIC_DIR': 'theme',
+        }
+        self.assertDictEqual(overrides, expected)