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 converter` makes:

`posts/foo/bar/2015-01-01-some_post.md`

Route to:

`posts/foo/bar/some_post/index.html`

(assuming `converter` strips the date prefix and markdown extension)

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.

Edit: updated usage based on @nagisa 's feedback.
Why not just `System.FilePath.dropTrailingPathSeparator`?