Edits task configuration.

            `env`
                Runtime ``Environment`` instance.
            `args`
                Arguments object from arg parser.

def execute(self, env, args):
        """ Edits task configuration.

            `env`
                Runtime ``Environment`` instance.
            `args`
                Arguments object from arg parser.
            """

        task_name = args.task_name

        if not env.task.exists(task_name):
            raise errors.TaskNotFound(task_name)

        if env.task.active and task_name == env.task.name:
            raise errors.ActiveTask

        # open in task config in editor
        task_config = env.task.get_config_path(task_name)

        if not _edit_task_config(env, task_config, confirm=True):
            raise errors.FocusError(u'Could not open task config: {0}'
                                    .format(task_config))