CriticMarkdown support

I have seen that there is some discussion about [annotations](https://github.com/hackmdio/codimd/issues/27). I think that a good light weight alternative would be to implement [CriticMarkdown](http://criticmarkup.com/) support. The extension is pretty [easy to learn](http://criticmarkup.com/users-guide.php) and I think will suffice basic editorial needs.

The only place where I will see some difficulties would be on nested long discussions about a particular piece of text, that is a task where [hypothesis](https://hypothes.is/) is better suited. Maybe some bridge between hypothesis and CodiMD via CriticMarkdown could be developed for that in the future.