Add Support for Jinja2 Overrides

[rarnold97]

Ideally, a user should be able to add in their own jinja file that can inherit from the jupyter lab template used in notebook.html.j2 The use case in my application was I wanted to be able to make cell outputs collapsable, and remove execution counts from the rendered cells. It is currently impossible to do so without forking or just making my own nbconvert plugin.

Example config:

plugins:
  - mkdocs-jupyter
      custom_overrides:
        my_jupyter_overrides/