Add a pages_generator_finalized signal.

I wrote a plugin that I'd like to also be able to run on pages in
addition to articles. Adding this signal will let me update the content
when a page is finished being generated.

pelican/signals.py

@@ -12,4 +12,5 @@ article_generator_finalized = signal('article_generate_finalized')
 get_generators = signal('get_generators')
 pages_generate_context = signal('pages_generate_context')
 pages_generator_init = signal('pages_generator_init')
+pages_generator_finalized = signal('pages_generator_finalized')
 content_object_init = signal('content_object_init')