Handle a setup.py entry point to let plugins add new kinto subcommands

[Natim]

In kinto-elasticsearch, kinto-signer and kinto-account we have commands that we would like to run with kinto COMMAND.

Because kinto-account is a build-in plugin we did it by coupling the plugin code with the kinto code 🏸
I would be better to use an entry-point so that installed plugin could register their subcommands.

• https://setuptools.readthedocs.io/en/latest/pkg_resources.html#entry-points
• https://stackoverflow.com/a/9615473/186202