Slug prefixes

Depending on the app that's built with Kilt, there might be some "ancillary" objects -- small CRUD objects that will populate dropdowns, provide groups, etc., but never be exposed as public URLs.  They're still Kilt objects, though, and they will be provided a unique slug.  

Their slug can get in the way, though.  Here's a story:

Setup:

```yaml
  page:
    fields:
      name: text
      body: tinymce
  blog_category:
    slug_prefix: 'blog-category'
    fields:
      name: text
```

> A user creates a blog category called "Alternative Energy"  The blog category (which will never be exposed as a page) is slugged as "alternative-energy".  Then the user creates a page called "Alternative Energy" with the intention of the page being exposed as a URL.  But since the blog category took up "alternative-energy", the public page gets the nasty-looking "alterative-energy-19287319273" URL.

I thought a quick way to take care of this potential problem is to allow a conditional ```slug_prefix``` value on an object.  If this slug prefix exists, and system-generated slugs will prepend ```slug_prefix``` to the slug.  So in the story above, the blog category would get "blog-category-alternative-energy' and the page would get "alternative-energy".

Thoughts, concerns?  I wanted to document this as well, but I didn't see a good place to do it beyond putting it in a default file.  Later on we should talk about documenting this app, I know it well now and think I can help.