Added support for markdown in heading cells when they are nbconverted.
Avoid calling of wrap function in headers of the markdown tpl when is nbconverted.
Added missing space.
We used a better wordwrap function provided by Jinja which let us wrap the titles in the correct way.
Do not wrap headers in markdown.tpl because inconsistency in the interpretation of multiline headers.
Merge pull request #3576 from damianavila/header_markdown

Added support for markdown in heading cells when they are nbconverted.