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

@minrk as posted a PR where provides the possibility to write markdown in heading cells: https://github.com/ipython/ipython/pull/3531

@Carreau has pointed out the need of a counterpart in the nbconvert machinery.

@jakobgager and @jdfreder have pointed out that the latex exporter already supported markdown in heading.

So, I have made some little changes for the remaining basic templates: basichtml, rst, markdown (python tpl don't need changes) to cover this issue from the nbconvert machinery point of view.

Damián.