Now this is still somewhat preliminary. But because there are now enough extensions, there's a new Plugin Manager for 2.1.8. See the manual section on it.

It basically just provides a list of available contrib/ plugins directly from the repository:

It needs to be enabled in the 🔌 Features tab first.

• Also allows to update existing/core plugins.
• Instantly activates and deactives channel plugins on saving (very experimental, but seems to work).
• User plugins end up in ~/.config/streamtuner2/plugins/

And this may become a Python module by itself later, https://pypi.python.org/pypi/pluginconf/.