Look up a templating engine within the current app and context. `engine` can be one of the following:  * a function: should conform to `function(content, data) { return interploated; }` * a template path: 'template.ejs', looks up the extension to match to the `ejs()` helper * a string referering to the helper: "mustache" => `mustache()`  If no engine is found, use the app's default `template_engine`
function(engine) {
      var context = this, engine_match;
      // if path is actually an engine function just return it
      if (_isFunction(engine)) { return engine; }
      // lookup engine name by path extension
      engine = (engine || context.app.template_engine).toString();
      if ((engine_match = engine.match(/\.([^\.]+)$/))) {
        engine = engine_match[1];
      }
      // set the engine to the default template engine if no match is found
      if (engine && _isFunction(context[engine])) {
        return context[engine];
      }

      if (context.app.template_engine) {
        return this.engineFor(context.app.template_engine);
      }
      return function(content, data) { return content; };
    }