Improve CD Core

Description

Improve CD Core in terms of API and structure to better support different scenarios.

• Add a simple way to extend *DefaultContentModelMapper *if you want to add and/or change a value in the model.
• Expose ContentModelMapperBase that inherits IContentModelMapper in a public namespace with additional methods or properties:
• Order *and *CanHandle: this pattern is about the same as converter, which allows you to select which
  mapper should be chosen based on a given content and order.
• As a result, we use a factory to decide which mapper to used to handle content in the controllers.

• Create a new class, ContentLanguageModel, that inherits LanguageModel. This class has the property “Link,” which lets users access content in a language by clicking the link. This class handles languages in CD instead of in LanguageModel like before.

• Expose IExcludeFromModelTypeRegistration. This allows a model type to not be registered by the default converters.