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:
I'm beginning to think that this `sluggify` function would turn out to be immensely useful throughout the rest of the project as well, and since it doesn't really have anything to do with Documents in the first place, would it be better to move it to Utils?