Mac: Remove quarantine flag from app

I encountered an issue with my favorite macos Markdown editor MacDown where macos Catalina was reporting the file as damaged. It turns out that Catalina has tightened the quarantine rules for apps, but that a command line fix was available, namely manually removing the quarantine flag for the app:

> xattr MacDown.app
com.apple.quarantine
> sudo xattr -r -d com.apple.quarantine MacDown.app
Password: