Automatically convert Jupyter notebooks to Markdown

[Geo]

Use the Python nbconvert module to clean up Jupyter notebooks and import them as blog posts.

Algo idea:

1. Recursively locate all Jupyter notebooks in a site's notebooks/ folder
2. Parse each notebook into a Markdown page bundle within content/post/ folder using nbconvert's Python function
   • defaults to the h1 heading (falling back to filename) as the Hugo page title, and the conversion date for date
   • If the first cell is Jupyter 'raw' type and begins with --- (YAML) use it as the page's front matter and hide that cell from the output
   • Fetch front matter options from notebook metadata field

Tests:

• Add a unit test on a Jupyter notebook and check the Markdown output

[John Muchovej]

I've worked with nbconvert in the past. However, are we to assume that exports should be done as-is, or would there be an intent to integrate some use-defined way to allow for things like the RemoveTagsPreprocessor to be used?

[Geo]

@ionlights thanks - I've updated the rough WIP algo above. Initially, lets keep it simple and only remove the first cell if it contains raw YAML front matter.

[Geo]

Implemented in 637ab31