Add the PATH_METADATA setting

Sometimes the base filename doesn't have everything you need.

Remember that os.sep is platform dependent, so using it in a regular
expression for this setting may not be portable (boo MS Windows!).

pelican/settings.py

@@ -77,6 +77,7 @@ _DEFAULT_CONFIG = {'PATH': '.',
                    'DEFAULT_ORPHANS': 0,
                    'DEFAULT_METADATA': (),
                    'FILENAME_METADATA': '(?P<date>\d{4}-\d{2}-\d{2}).*',
+                   'PATH_METADATA': '',
                    'FILES_TO_COPY': (),
                    'DEFAULT_STATUS': 'published',
                    'ARTICLE_PERMALINK_STRUCTURE': '',