API Added link-to-file ability

Added ability to link a 'user template' to a template file defined on the
filesystem. These template files will be used instead of the raw Content
provided by the user

Requires developers to define these templates in the specific
directory {theme_name}/user-templates
Added composer.json
Added ability to specify action specific templates for a particular template so that overrides are not just limited to 'index' actions for a page