Add comment function
          #92
        
          
      
                
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It's a rather uncommon use-case, but we had someone in the elm-lang slack asking for this, and they actually had a valid use-case: copy pasting Elm generated HTML into an e-mail template, which - for some insane reason - requires comments.
This is unlikely to have any averse affects, so I figured I'd PR it.
Closes #43