Add articles with subparts.

Sub-articles are useful when an article has logically related parts
that are too large to be conveniently included in a single article,
yet are not independent enough to merit a separate article.

The site pxquim.pt uses sub-articles to associate multiple photo galleries
with a single article. Each photo gallery goes into a sub-article.

By convention, subparts of articles are articles with "--" in their name.
The part before the "--" should be the slug of the parent article.

For example:

* article.md        (the parent article)
* article--one.md   (a sub-article of "article.md")
* article--other.md (another sub-article of "article.md")

The article object gets a list of subparts[].

The sub-article object gets the following attributes parent:

* parent -- Reference to the parent article object.
* title -- Changed to start with the parent article plus a comma.
* subtitle -- The original title of the sub-article.

pelican/generators.py

@@ -346,7 +346,7 @@ class ArticlesGenerator(CachingGenerator):
 
     def generate_articles(self, write):
         """Generate the articles."""
-        for article in chain(self.translations, self.articles):
+        for article in chain(self.translations, self.articles, self.article_subparts):
             signals.article_generator_write_article.send(self, content=article)
             write(article.save_as, self.get_template(article.template),
                   self.context, article=article, category=article.category,
@@ -536,6 +536,30 @@ class ArticlesGenerator(CachingGenerator):
 
         signals.article_generator_pretaxonomy.send(self)
 
+        slugs= {}
+        for article in self.articles:
+            slugs[article.slug]= article
+        self.article_subparts= []
+        articles_main= []
+        for article in self.articles:
+            if '--' in article.slug:
+                (pslug, _)= article.slug.split('--', 1)
+                if pslug in slugs:
+                    parent= slugs[pslug]
+                    if not hasattr(parent, 'subparts'):
+                        parent.subparts= []
+                    parent.subparts.append(article)
+                    article.subpart_of= parent
+                    self.article_subparts.append(article)
+                    article.subtitle= article.title
+                    article.title= article.title + ", " + parent.title
+                else:
+                    logger.error('No parent for {}'.format(pslug))
+                    articles_main.append(article)
+            else:
+                articles_main.append(article)
+        self.articles= articles_main
+
         for article in self.articles:
             # only main articles are listed in categories and tags
             # not translations
@@ -547,6 +571,13 @@ class ArticlesGenerator(CachingGenerator):
             for author in getattr(article, 'authors', []):
                 if author.name != '':
                     self.authors[author].append(article)
+
+        for article in self.article_subparts:
+            # But parts should also show in tags
+            if hasattr(article, 'tags'):
+                for tag in article.tags:
+                    self.tags[tag].append(article)
+
         # sort the articles by date
         self.articles.sort(key=attrgetter('date'), reverse=True)
         self.dates = list(self.articles)
@@ -590,7 +621,8 @@ class ArticlesGenerator(CachingGenerator):
         self.authors.sort()
 
         self._update_context(('articles', 'dates', 'tags', 'categories',
-                              'tag_cloud', 'authors', 'related_posts'))
+                              'tag_cloud', 'authors', 'related_posts',
+                              'article_subparts'))
         self.save_cache()
         self.readers.save_cache()
         signals.article_generator_finalized.send(self)