Design a plugin eco-system

[gavinhenderson]

We should create a 'plugin ecosystem'.

This would mean we can add plugins like a Spotify plugin really easily, but it also means we don't clutter the settings with Spotify options for users unless they enable the Spotify plugin.

It will also give us scope to add other Spotify competitors or other plugins much easier.

A plugin should be able to create a tree on the fly based on network data. It should then be able to trigger events on selection of each key.

We really need to consider security. We don't want people sending off secrets to servers.

Outcome:

• A design of how the plugin ecosystem will work
• A list of issues to make it happen

Ref: #241