Install nbextensions via `setup.py`

[peteut]

As suggested by @oschuett, nbextensions shall be installed via setup.py. Refer to widget-cookiecutter for a template.

[peteut]

See also migration_guides.

[lsdch]

Hi ! Currently working on extending your widget to support multiple file uploads #5. To start on a clean base I used widget-cookiecutter as suggested. Working code is available here (it's an exact clone regarding features, no multiple file uploading yet).

I'm a newbie at contributing to open-source projects. I know the fork+PR way is preferred but it was easier to let cookiecutter build the whole structure from scratch instead of going through all them copy-pastes. Let me know if it helps anyway !