Basic audio diff UI

This is a first pass at the CLI we've been imagining/sketching out all along where we show representations of file/diff aligned above each other. There's absolutely no attempt at audio playback to help comparison yet, but there *is* an attempt to make the UI even driven so that we can easily start to hook in more interactivity. The basic shape feels right, but I expect the layout/interaction code will get complicated and necessitate some reorganisation fairly promptly!