feature: Add support for python markdown and image attachments

[jaycode]

I updated the code to work with Python Markdown extension:

https://github.com/ipython-contrib/jupyter_contrib_nbextensions/tree/master/src/jupyter_contrib_nbextensions/nbextensions/python-markdown

It will likely work with other Jupyter extensions as well since it uses jupyter nbconvert shell command to get content now.

I also adjusted the code so that the system copies all images to output directory. It does not currently work with attachments since I do not use them myself. To include image, use the following Markdown syntax:

![image](image.url.jpg)

[jaycode]

Issue reference:
#86

[danielfrg]

Thanks for the PR! I would like to support those extensions. I have not updated this in a while so i have a few questions.

Does it still work with the CSS tricks I am using? Is the HTMLExporter is now basically ignored? I would love to find a way to support the extensions without the explicit subcall. I will try to test this soon.