On a given interface store which models implement it

[penard-cb]

Describe your PR and link to any relevant issues.

On any given model we know of which interface it is. However there are cases where if you're writing some custom models, you would like to know what a type is implemented by:

For example:

type EntryCollection {
  total: Int!
  skip: Int!
  limit: Int!
  items: [Entry]!
}

interface Entry {
  sys: Sys!
  metadata: metadata!
} 

type Page implements Entry {
  sys: Sys!
  metadata: metadata!
  linkedFrom(allowedLocales: [String]): PageLinkingCollections
  name(locale: String): String
  ...
}
type PageArticle implements Entry {
  sys: Sys!
  metadata: metadata!
  linkedFrom(allowedLocales: [String]): PageArticleLinkingCollections
  name(locale: String): String
  ...
}

For a Page and a PageArticle we'll know it implements Entry. But if we want to add additional model functionality for an interface, we don't know all the types that implement it. This PR makes it possible to add complex model definitions and helpers (marshalers/unmarshalers) for the Entry type.

[coveralls]

coverage: 75.766% (+0.02%) from 75.75% when pulling cf5f468 on penard-cb:feat/add-model-object-reference-to-interface into 0eb95dc on 99designs:master.

[StevenACoffman]

Thanks! I'm loving these improvements!