Support raw_html extension in commonmark reader

[jgm]

The commonmark writer supports raw_html, but the reader does not, which is a bit awkward.

Because raw HTML parsing is a core feature of commonmark, it can't easily be disabled in the parser with an extension. In principle we could do it, though, by avoiding the use of defaultSyntaxSpec and instead building this up ourselves, omitting the raw HTML parsers if raw_html isn't set. This is probably worth doing, though it might create problems down the road if there are changes to defaultSyntaxSpec in commonmark-hs.