Move additional CSS/JS to separate files, include only in article pages.

I love this plugin and plan to use it for my site. To do so, I adjusted few things that bugged me. I don't know if these are generally wanted features or not - merge if wanted!

Issues it addresses:
- The current version puts about 1600 lines of mainly css and some javascript into the html-header.
- It includes this massive header on each page.
- Site-specific css are hard-coded in the plugin.

The changes address the issues in the following way:
- The additional css/js is put into separate files in the /theme/css and /theme/js directories.
- The required header is only on the articles pages, and only if the tag 'ipynb' is present.
- Site-specific css-changes have to be saved outside the plugin in 'output/theme/css/ipynb_mod.py'. 

I am not familiar enough with the pelican code itself. The following could therefore be improved: The paths 'output/theme/css' and 'output/theme/js' are hard-coded. Pelican has, presumably, variables containing these paths. These variables should be used instead.

Thanks for the great plugin!
Dieter