I've run the black code formatting tool against everything:
black tests datasette setup.py
I also added a new unit test, in tests/test_black.py, which will fail if the code does not
conform to black's exacting standards.
This unit test only runs on Python 3.6 or higher, because black itself doesn't run on 3.5.
Uses https://pluggy.readthedocs.io/ originally created for the py.test project
We're starting with two plugin hooks:
prepare_connection(conn)
This is called when a new SQLite connection is created. It can be used to register custom SQL functions.
prepare_jinja2_environment(env)
This is called with the Jinja2 environment. It can be used to register custom template tags and filters.
An example plugin which uses these two hooks can be found at https://github.com/simonw/datasette-plugin-demos or installed using `pip install datasette-plugin-demos`
Refs #14
