Test for FEED & FEED_RSS before generation.

Then we bypass all feed generation of both are set to None or throw a warning if SITEURL is unset.
Updated all documentation, to make sure the usage and warning is clear.

pelican/generators.py

@@ -124,6 +124,12 @@ class ArticlesGenerator(Generator):
 
     def generate_feeds(self, writer):
         """Generate the feeds from the current context, and output files."""
+        if self.settings.get('FEED') is None and self.settings.get('FEED_RSS') is None:
+            return
+        elif self.settings.get('SITEURL') is '':
+            logger.warning(
+                'Feeds generated without SITEURL set properly may not be vaild'
+            )
 
         if self.settings.get('FEED'):
             writer.write_feed(self.articles, self.context,

pelican/settings.py

@@ -25,6 +25,7 @@ _DEFAULT_CONFIG = {'PATH': '.',
                    'CATEGORY_FEED': 'feeds/%s.atom.xml',
                    'TRANSLATION_FEED': 'feeds/all-%s.atom.xml',
                    'FEED_MAX_ITEMS': '',
+                   'SITEURL': '',
                    'SITENAME': 'A Pelican Blog',
                    'DISPLAY_PAGES_ON_MENU': True,
                    'PDF_GENERATOR': False,