This change introduces a new plugin hook, publish_subcommand, which can be
used to implement new subcommands for the "datasette publish" command family.
I've used this new hook to refactor out the "publish now" and "publish heroku"
implementations into separate modules. I've also added unit tests for these
two publishers, mocking the subprocess.call and subprocess.check_output
functions.
As part of this, I introduced a mechanism for loading default plugins. These
are defined in the new "default_plugins" list inside datasette/app.py
Closes#217 (Plugin support for datasette publish)
Closes#348 (Unit tests for "datasette publish")
Refs #14, #59, #102, #103, #146, #236, #347
