HTML comments should become Clojure comments

[mtnygard]

Minimal test case

<p>
  <!-- This is a comment -->
</p>

outputs

[:p  "<!-- This is a comment -->" ]

The comment was converted into content, which is never the right answer. It would be more helpful to convert it to a Clojure comment, or if that would choke some of the templating systems, just drop the comment altogether.