Add a slugify Liquid filter

[kansaichris]

In #2864, @alfredxing noted:

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. 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?

[parkr]

Looks great, thanks @kansaichris!

[alfredxing]

Awesome, thanks for working on this!

[kansaichris]

@imathis In case you're interested, this is the slugify filter that I was working on. 😄