Proposed addition to friendly_id guide

User editable slug - with defaults

User editable slug - with defaults
----------------------------------

Case: 
We want our post class to have slug and title attribues that are editable by the user.
The user should be able to edit the title and slug independently.
The slug should default to a slugged version of the title.

example output:
```ruby
post = Post.create(title: "How Long is a Long Long Time", slug: 'how-long')
post.slug 
# 'how-long'
post = Post.create(title: "How Long is a Long Long Time")
post.slug 
# 'how-long-is-a-long-long-time'
```
```ruby
class Post < ActiveRecord::Base
  extend FriendlyId
  friendly_id :title, :use => [:slugged]
  validates_prescence_of :title

  def should_generate_new_friendly_id?
    if !slug?
      name_changed?
    else
      false
    end
  end
end
```