Modular architecture

From @nelsonic

> Note: I had a quick chat with others regarding the fact that this PR including all the functionality in a single repo without splitting things out into independently tested plugins. This is fine for an MVP but we need to get to the point where we composing from smaller pieces. see: https://en.wikipedia.org/wiki/Unix_philosophy

This issue is to discuss migrating the architecture of the app away from a monolith and towards something more modular (probably with a hierarchy of components).