MAINT: Install cmdline tool when niviz is pip installed

[jerdra]

Niviz provides command-line utilities in order to generate reports and SVGs under the make_svgs.py script. See:

https://python-packaging.readthedocs.io/en/latest/command-line-scripts.html

https://setuptools.pypa.io/en/latest/userguide/entry_point.html

https://medium.com/nerd-for-tech/how-to-build-and-distribute-a-cli-tool-with-python-537ae41d9d78

For references!

This will also be useful for implementing our schema validator - related to #13