Re-work as a proper standalone Python package

Add a setup.py so that installation is easier and a standalone CLI
script called csv-metadata-quality is provided. Now the user only
needs to run this from a virtual environment inside the project
directory:

    $ pip install .

Eventually I could publish this on PyPi when I settle on a more
appropriate package name.

See: https://packaging.python.org/tutorials/packaging-projects/
See: https://chriswarrick.com/blog/2014/09/15/python-apps-the-right-way-entry_points-and-scripts/

csv_metadata_quality/__main__.py

@@ -1,5 +1,9 @@
 from csv_metadata_quality import app
 from sys import argv
 
+def main():
+    app.run(argv)
+
+
 if __name__ == '__main__':
-    app.main(argv)
+    main()