Add a slugify Liquid filter

In https://github.com/jekyll/jekyll/pull/2864, @alfredxing [noted](https://github.com/jekyll/jekyll/pull/2864#discussion_r16967929):

>I'm beginning to think that a `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?

[I agreed](https://github.com/kansaichris/jekyll/commit/3ca1245027547169ff70d996e076ed06e8fab99f). I think we can do even better, though, and provide this functionality as a Liquid filter.

@parkr @gjtorikian @alfredxing What do you think? Could you suggest some more strings to test to ensure that this feature works as expected?