docs: def register(cls, name, klass, content_type='text/plain'):
          """
          Register an emitter.
  
          Parameters::
           - `name`: The name of the emitter ('json', 'xml', 'yaml', ...)
           - `klass`: The emitter class.
           - `content_type`: The content type to serve response as.
          """
          cls.EMITTERS[name] = (klass, content_type)