Improve IPYNB template to avoid HTML tags

Use kramdown `markdown="1"`:

    If an HTML tag has an attribute markdown="1", then the
    default mechanism for parsing syntax in this tag is used.
    (http://kramdown.gettalong.org/syntax.html)
Update Markdown from IPYNB based on new template
Merge pull request #652 from r-gaia-cs/clean-markdown-from-ipynb

Small clean at Markdown from IPYNB