[WIP] prototype a keyboard shortcuts plugin

[aeschylus]

This represents some discussion about possible plugin architectures and initialisation methods. Not to be merged.

[christopher-johnson]

For example, what about something like this?

var myMirador = new MiradorViewer(config);
myMirador.registerPlugins([keyboardPlugin, rulerPlugin]);

This looks like a God Object.

Fusion.js might be interesting if you want to consider a DI based plugin architecture.

Docs on how to create a plugin: https://fusionjs.com/docs/guides/creating-a-plugin

This describes it: https://eng.uber.com/fusionjs/