plugins: load plugins from setuptools entry points

[Exirel]

By defining a sopel.plugins entry point in a setup.cfg (or setup.py) it is now possible to expose a Sopel plugin:

setup(
    name='my_plugin',
    version='1.0'
    entry_points={
        'sopel.plugins': [
            'custom = my_plugin.path.to.plugin',
        ],
    }
)

This definition will allow Sopel to load the custom plugin, from the Python sub-module my_plugin.path.to.plugin from the my_plugin package.

I'm not 100% happy with my abstractclass/subclasses, but I think that's for another day. Here, I wanted to showcase how quick and easy it is to implement new ways to discover Sopel plugins.

I'd like to mention that, in my opinion, entry points are probably a better option than sopel_modules.* subpackages.

[dgw]

I really like this. It's much more elegant than the sopel_modules namespace introduced under my predecessor (and not only because it should have been named sopel_plugins 😛)! Just some nitpicky (as usual) naming stuff from me—and only one grammar fix! You're getting better. 😸

[Exirel]

Just a quick note: fixed as asked. It looks good for a last review & merge. Can't wait to play with that feature for future plugins!

[dgw]

One last English nitpick. Don't worry about the EntryPointPlugin class note; I'm just asking, not requesting that you update or rewrite anything for this PR.

[Exirel]

@dgw done! 😉

[dgw]

Hoorah!