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).

No need to apologize. I've been doing this long enough to know that PRs are the *start* of a conversation, not take it or leave it.

> - It surely can be generalised to something like `indexRoute :: (String -> String) -> Route` ...
> - It assumes certain preconditions are upheld. These are not guaranteed by hakyll.

Assuming point one is addressed, what is left in point 2?