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.
Sorry to tell you this, but in my opinion this doesn’t look good enough to qualify for merging into hakyll (though being just a passerby contributor I have no power behind my words).

Here are some of the issues I see with this implementation:
* It surely can be generalised to something like `indexRoute :: (String -> String) -> Route` where the first argument converts filename into a directory name; (this would also unload most of my other nitpicks to end user who can then only handle their own use cases)
* It assumes certain preconditions are upheld. These are not guaranteed by hakyll.