critic markup integration

[semiopat]

As a collaborative tool, it could be great that HackMD | CodiMD implement collaborative modification provides by critic markup, as described here, at least the five basic critic marks:

Addition {++ ++}
Deletion {-- --}
Substitution {~~ ~> ~~}
Comment {>> <<}
Highlight {== ==}{>> <<}

That could be a huge enhancement for collaborative writing…