add a parse string function to enable #<> in code editor

Now the code like "# '<' tag '>' '<' tag '>'" in code editor will disappear after submission.

This issue is important because students are using comment to answers questions.

By default, Codemirror will not decorate comment part. Fixed this by pre-parse the comment part, and change '<'  in comments to '<'span'>' < '<'/span'>'