Update _config.yml
Update _config.yml

1. Separate `powered`, `theme` and `custom` options.
2. Refactor some options and comments & add template to `custom_text`.
chaoskyme/hexo-theme-next,iissnan/hexo-theme-next,mchsinx/hexo-theme-next,chaoskyme/hexo-theme-next,geekrainy/hexo-theme-next