Add 'title' to collection URLs

Spiritual successor to https://github.com/jekyll/jekyll/pull/2799.

This addresses @parkr's [comment](https://github.com/jekyll/jekyll/pull/2799#issuecomment-53336212) from https://github.com/jekyll/jekyll/pull/2799:

>I'd like them to map to Liquid variables (i.e. `page.name` == `:name`, etc). Ideally, we'd have:
>
>1. `name` == `File.basename(path, ".*")`
>2. `title` == `page.title` in Liquid, sluggified

This also addresses @gjtorikian's [comment](https://github.com/jekyll/jekyll/issues/2847#issuecomment-53826395) from https://github.com/jekyll/jekyll/issues/2847:

>Oh dang, I didn't know `:title` slugified.
>
>Yeah, on documents would be grand.

Let me know what you think! :heart:
Thank you!

If you want to add it to `Jekyll::Filters`, go ahead :smile: I think that would be a valuable add. It can just pass to `Jekyll::Utils.slugify`.