Warn when markdown not selected but ```d found

[ntrel]

E.g. this post is hard to read without formatting:
https://forum.dlang.org/post/vbda5s$fng$1@digitalmars.com

[CyberShadow]

The idea makes sense on its own, but

https://forum.dlang.org/post/vbda5s$fng$1@digitalmars.com

That post was not created using DFeed.

[ntrel]

OK. Perhaps it would be good to allow the reader to manually switch to markdown? A hint could be displayed when ```d is present in a non-markdown post.