Extends #1148: article modification date from FS.

- This pull request adds a new attribute to the article object: mdate.

  - article.mdate retrives the article file last modification date from the
    fylesystem (os.stat(full_path)), thus providing article modification date
    automatically.

  - This extends #1148 in the sense that an author doesn't have to add the
    :modified: metadata to each article, as she can just edit the template
    files to display the new article attribute.

  - Documentation updated to describe the new feature.

docs/themes.rst

@@ -218,6 +218,8 @@ Variable        Description
 =============   ===================================================
 article         The article object to be displayed
 category        The name of the category for the current article
+mdate           The article modification date and time as it
+                appears in the filesystem
 =============   ===================================================
 
 Any metadata that you put in the header of the article source file

pelican/readers.py

@@ -544,6 +544,8 @@ def path_metadata(full_path, source_path, settings=None):
                 os.stat(full_path).st_ctime)
         metadata.update(settings.get('EXTRA_PATH_METADATA', {}).get(
             source_path, {}))
+    metadata['mdate'] = SafeDatetime.fromtimestamp(
+        os.stat(full_path).st_mtime)
     return metadata