Support Markdown extension: Alerts

[lidel]

GitHub added some CSS for styled Alert blocks:

https://github.blog/changelog/2023-12-14-new-markdown-extension-alerts-provide-distinctive-styling-for-significant-content/

[chrishrb]

Hi, I think that grip is no longer maintained, so I created my own version of it in go - without the need of the github api. It also supports alert blocks!
Check it out: https://github.com/chrishrb/go-grip. Feel free to raise issues or feature requests!