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.
How strong is that convention/recommendation? I thought I had seen other functions documenting it as an assumption for their use.

Assuming it's a strong convention, what about providing this `indexRoute` which relies on it (and documents it) and a separate `indexRouteWith` which takes your `String -> String` argument for those that don't follow it. I see this `fooWith` convention followed pretty regularly throughout some of the Hakyll extensions (e.g. tags).

If it's not a strong convention at all, then I'm in agreement that only `indexRoute` should be provided and it should take an argument of `String -> String` to handle whatever filenames users are using.