[Feature] Implement GitHub style highlighted blockquotes

[LittleGitPhoenix]

Since May 10th 2022 GtiHub supports highlighting important messages via custom annotated blockquotes. I do require to emphasize critical information within README or CHANGELOG files quiet often. Currently I am using custom html for this but sadly GitHub will not render those elements properly leaving user experience somewhat lacking.

Typora would render those special block quotes like this:

GitHub like this:

Hopefully this can be implemented in a future update, so that Typoras rendering of those special block quotes matches GitHubs and they can be used throughout markdown files consistently.

[LittleGitPhoenix]

Since I contacted the developer via mail before raising this issue, he contacted me directly stating that the newest development version of Typora (1.8.0-dev) does already support the new annotated blockquotes and that those changes will be ported into the stable version sometime soon. Good enough for me.