Add support for nbextensions and image attachments

[jaycode]

Some nbextensions modules are very useful, like python-markdown that allows you to include inline Python code in Markdown cells.

I have added a support for nbextensions in my fork of pelican-ipynb. One problem though is that it uses shell command to run the conversion code. Let me know if there is a better way to do this or otherwise merge my fork.

I have also added a feature to copy attachment images into the output directory.