Regression: HTML comments get escaped

**Symptom:**

1. Turn on Cryogen "previews" in a new installation (`:previews? true`)
2. Add `` after one paragraph in one post
3. `` is escaped and rendered to the user. Furthermore, it does not restrict snippet length.

**Reason:**

Snippet trimming function expects `` to be parsed as an HTML comment. (See `more-marker?`) However, this is what it gets: `{:tag :p, :attrs nil, :content ()}`

**Tried:**

I printed the document at various stages of rendering, and found that the comment comes out already escaped from `render-fn` (as defined in `cryogen-flexmark` plugin).

Flexmark does have a couple different flags related to comments and escaping, which I tried setting to false (even though they should be false by default). This didn't help. There don't seem to be any open issues for HTML comments in the flexmark project.

**Next:**

Be good if someone else read the flexmark documentation again, I might have missed the relevant flag combination.

You could also look at the `Document` object that comes out of `Parser.parse` and see if it parsed the comment correctly. This would narrow down the flag search to either parsing or rendering. Unfortunately I don't have the environment set up for this at the moment.