Plugins support

[wizzomafizzo]

I want Core to suppose user-writeable and loadable plugins. How this will work exactly still needs to be worked out but that's the idea at a high level. I'd like them to be able to support out of scope things like: replacing the BGM script, editing the mister ini file, choosing background etc.

I'm pretty sure I want plugins to be written in JS and loaded using the Goja library.