Helpers for prettier links by relying on auto-index

Add a route and context helper to make it easier to have "pretty" links.

For example, `route indexRoute` makes:

`posts/foo/bar/2015-01-01-some_post.md`

Route to:

`posts/foo/bar/some_post/index.html`

Then, `indexedUrlField "url"` makes:

`$url$`

Show:

`posts/foo/bar/some_post/`

In your template.

So long as auto-index is enabled on your server, visiting this URL will render the generated index.html.
While `YYYY-MM-DD-c` is a recommendation, people are not required to name their files exactly this way. What happens when  people name their files `just-filename.ext` way? 

Though a bit of a stretch, what happens to people who use filesystems differently and place no emphasis on the part after last dot in the filename (i.e. `YYYY-MM-DD-this.is.my.awesome.blog.post`)? Extensions are pretty much a DOS/Windows thing which people began using elsewhere because it is convenient. 

Other corner cases I can think of and which should have a predictable/documented behaviour would be:
* filename before the “extension” is empty (`.md`), i.e. it is a dot file.
* YYYY-MM-DD-.md
* YYYY-MM-DD.md