Add `Unsafe flag for HTML rendering

[pablo-meier]

🎵 The song for this PR is Gerudo Valley Theme Acapella, from the Legend of Zelda: Ocarina of Time soundtrack, performed by Smooth McGroove. 🎵

I'm having trouble rendering raw HTML from my Markdown using Cmark, even when omitting the Safe flag:

Cmark.of_string "**Hello World**<img src=\"html\" />" |> Cmark.to_html ~flags:[];;
> : string = "<p><strong>Hello World</strong><!-- raw HTML omitted --></p>\n"

This makes it hard to render the content I'd like to render.

Looking at other cmark wrappers, it appears the flag 131072 can explicitly set "Unsafe" (source).

1 lsl 17;;
> : int = 131072

Haven't tested this since I haven't installed car or anything, but thought I'd try pushing it up before anything else.

[pablo-meier]

Also have a branch, where I moved it over to Dune, because I'm still a little new to OCaml and that's the only build tool I'm even a little familiar with 😛