Allow dynamic link names.

[dblock]

A model that has image_versions might want to have a link for image item in the field. Like so:

{
  "id" : 123,
  "image_versions" : ["small", "large"],
  "_links" : {
     "image:small" : {
       "href" : "http://example.org/images/123/small.jpg"
     },
     "image:large" : {
       "href" : "http://example.org/images/123/large.jpg"
     }
   }
}

we can override https://github.com/apotonick/roar/blob/master/lib/roar/hypermedia.rb#L54, but maybe we want a better DSL? How about something that is evaluated on an instance before representing it?

before_represent do
  represented.image_versions each do |image_version|
    link image_version do 
       "http://example.com/images/#{represented.id}/#{image_version}.jpg"
    end
  end
end