Rendering architecture rewrite part 2
    * Renamed renderers to template engines (closer to the concept)
    * introduced a delegating template engine
    * separated Freemarker from core and made it optional
    * tags do not require the full list of docs in RAM anymore
    * renamed org.jbake.parser.Engines.properties to MarkupEngines.properties
    * "index", "archive" and "feed" are normal document types (config options are now optional)
Rendering architecture: support for Groovy template engine
   * added GroovyTemplateEngine
   * GroovyRendererTest test case uses .gsp files equivalent to .ftl