autogenerate index.html for (or whatever the template: says) 

I need intermediate url's to be have valid html - a TOC is currently enough. for example, ``http://mkdocs.org/about/`` is not a topic, but someone may go there when typing the url.


It looks like some ``nav.py`` code has been broken - these pages are now listed, so I added a ``**HIDDEN**`` title to trigger my code


- I can raise a new PR to put this back if you like (now that i grok that code a little better)
```
                # Page config lines that do not include a title, such as:
                #    - ['index.md']
                # Will not be added to the nav items hierarchy, although they
                # are included in the full list of pages, and have the
                # appropriate 'next'/'prev' links generated.
```

``title=**HIDDEN**`` makes the page not listed in the nav items hierarchy, __and__ takes it out of the next/prev flow. however, those pages do have valid next/prev - so if you land on one, you can get out :).

If the ``.md`` file doesn't exist, then its generated using autoindex.html (ok, should really call it autoindex.md :) - and like the sitemap, might default it in a common dir, and allow over-ride in themes)

@tomchristie - would you merge this (with doc, cleanups and tests etc) or should I make it into a personal branch?
We should can't a more specific Exception here.