support comments by converting into special tags

we turn all the comments into <_comment> tags, and then just have a
converter that handles them. this way we avoid doing isCommentNode
checks all over the place.

lib/converters.coffee

@@ -222,6 +222,10 @@ module.exports = [
     surroundingBlankLines: true
     replacement: (content) -> content
   }
+  {
+    filter: '_comment'
+    replacement: (content) -> "<!-- #{content} -->"
+  }
   {
     filter: fallback
     surroundingBlankLines: true

lib/index.coffee

@@ -9,9 +9,12 @@ treeAdapter = require './tree-adapter'
 {cleanText, decodeHtmlEntities, isBlock, isVoid} = require './utils'
 
 {
+  createElement
   detachNode
   getCommentNodeContent
   getTextNodeContent
+  insertBefore
+  insertText
   isCommentNode
   isElementNode
   isTextNode
@@ -80,6 +83,13 @@ fixHeaders = (dom, ensureFirstHeaderIsH1) ->
     i++
   return
 
+convertCommentNode = (node) ->
+  commentElement = createElement('_comment', null, [])
+  insertText(commentElement, getCommentNodeContent(node))
+  insertBefore(node.parent, commentElement, node)
+  detachNode(node)
+  return commentElement
+
 ###*
  * Flattens DOM tree into single array
 ###
@@ -90,6 +100,7 @@ bfsOrder = (node) ->
     elem = inqueue.shift()
     outqueue.push elem
     for child in elem.childNodes
+      if isCommentNode(child) then child = convertCommentNode(child)
       if isElementNode(child) then inqueue.push child
 
   outqueue.shift() # remove root node