feat: 支持添加自定义内容用于follow认证 & 支持封面图 #36

[forrany]

尝试解决 #36

1. 添加自定义内容，追加在最后，用于 follow 订阅认证

2. 对于封面图无法识别的问题，follow会自动抓取内容的第一个图片作为封面，因此尝试通过meta抓取封面图

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