Write nbinteract converter for Gitbook

[SamLau95]

I'd like to replace all the convert_notebooks_to_html_partial.py files we have across these textbooks:

• https://github.com/data-8/textbook/blob/gh-pages/convert_notebooks_to_html_partial.py
• https://github.com/SamLau95/nbinteract/blob/master/docs/convert_notebooks_to_html_partial.py
• https://github.com/DS-100/textbook/blob/master/convert_notebooks_to_html_partial.py
• https://github.com/prob140/textbook/blob/gh-pages/nb2html.py

With something like:

$ nbinteract --gitbook foo.ipynb bar.ipynb

# converts all notebooks in folder and outputs to notebooks-html/
$ nbinteract --gitbook --output notebooks-html notebooks/

# converts all notebooks in folder, outputs to notebooks-html/,
# and outputs images to notebooks-images/
$ nbinteract --gitbook --output notebooks-html --images notebook-images notebooks/

# converts all notebooks in folder and recurses into all subfolders
$ nbinteract --gitbook -r notebooks/

# converts all notebooks in folder and recurses into all subfolders,
# then outputs notebooks into notebooks-html without folders
$ nbinteract --gitbook --output notebooks-html -r notebooks/

Here's a docopt docstring that I think might work:

'''Converts notebooks to interactive HTML pages or Gitbook pages.

Usage:
  nbinteract NOTEBOOKS...

Arguments:
  NOTEBOOKS  The list of notebooks or folders containing notebooks to convert.

Options:
  -g --gitbook               Convert to HTML partial for Gitbook
  -r --recursive             Recursively convert notebooks in subdirectories.
                             Leaves HTML files in the same folders as the
                             original notebooks.
  -o FOLDER --output=FOLDER  Outputs HTML files into FOLDER. If -r is also set,
                             will not recreate input folder structure in output
                             folder (all output files are direct children of
                             FOLDER).
  -i FOLDER --images=FOLDER  Extracts images from HTML and writes into FOLDER.
                             Requires -o option to be set as well.
'''

Right now we have the following CLI option:

$ nbinteract my_notebook.ipynb

Which is a simple alias of

$ jupyter nbconvert --to interact my_notebook.ipynb

We'll have to change the script to use Python instead of bash.

[SamLau95]

Closed in #48