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()

csv_metadata_quality/app.py

@@ -15,7 +15,7 @@ def parse_args(argv):
     return args
 
 
-def main(argv):
+def run(argv):
     args = parse_args(argv)
 
     # Read all fields as strings so dates don't get converted from 1998 to 1998.0