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/__init__.py

@@ -23,7 +23,7 @@ from pelican.plugins import signals
 from pelican.plugins._utils import load_plugins
 from pelican.readers import Readers
 from pelican.server import ComplexHTTPRequestHandler, RootedHTTPServer
-from pelican.settings import read_settings
+from pelican.settings import coerce_overrides, read_settings
 from pelican.utils import (FileSystemWatcher, clean_output_dir, maybe_pluralize)
 from pelican.writers import Writer
 
@@ -230,6 +230,18 @@ class PrintSettings(argparse.Action):
         parser.exit()
 
 
+class ParseDict(argparse.Action):
+    def __call__(self, parser, namespace, values, option_string=None):
+        d = {}
+        if values:
+            for item in values:
+                split_items = item.split("=", 1)
+                key = split_items[0].strip()
+                value = split_items[1].strip()
+                d[key] = value
+        setattr(namespace, self.dest, d)
+
+
 def parse_arguments(argv=None):
     parser = argparse.ArgumentParser(
         description='A tool to generate a static blog, '
@@ -323,6 +335,18 @@ def parse_arguments(argv=None):
                         help='IP to bind to when serving files via HTTP '
                         '(default: 127.0.0.1)')
 
+    parser.add_argument('-c', '--setting-overrides', dest='overrides',
+                        help='Specify one ore more SETTING=VALUE pairs '
+                             '(without spaces around =) to override '
+                             'settings files. If VALUE contains spaces, add quotes: '
+                             'SETTING="VALUE". '
+                             'Integers and strings are autoconverted, other values '
+                        'can be passed in in json notation, '
+                             'e.g. SETTING=none',
+                        nargs='*',
+                        action=ParseDict
+                        )
+
     args = parser.parse_args(argv)
 
     if args.port is not None and not args.listen:
@@ -358,6 +382,7 @@ def get_config(args):
     if args.bind is not None:
         config['BIND'] = args.bind
     config['DEBUG'] = args.verbosity == logging.DEBUG
+    config.update(coerce_overrides(args.overrides))
 
     return config