Enabling overriding of view and translation keys

* Adding ViewPathFragment conversion method

  I have been overworking the TranslationKeyFragment method for both
  View and Translation related information. I want to separate that
  concern and tease apart a solution for :render_as.

* Replacing usage of TranslationKeyFragment()

  Adding usage of ViewPathFragment() function. The underlying data
  structures delegate :to_view_path_fragment to
  :to_translation_key_fragment.

* Adding database columns for fragments

  Adding the :view_path_fragment and :translation_key_fragment columns
  to allow even greater customization of rendering options.

* Allowing override of data definition attributes

  Both :view_path_fragment and :translation_key_fragment are
  overridable.
Merge pull request #16 from ndlib/issue-12-render-as

Enabling overriding of view and translation keys