make slug field required

[m-henderson]

This will fix #49 but it does require the use to supply a slug.

[m-henderson]

@madskristensen if you don't want the slug field to be required, we can automatically generate one.

[tbasallo]

I've been meaning to auto-generate one based on the title in my fork. I think this would be ideal.

I wonder if there's a reason it wasn't considered.

[jimmylewis]

I think #52 is the proper fix for this issue. Slug shouldn't be marked as required, as it is normally supposed to be generated from post.Title.