Figure out plugins

[zaviermiller]

Creators of plugins should be able to:

• manage state (either for the app, their plugin, or the component)
• easily implement from existing go projects (hopefully normal go projects just work with zephyr, but idk)

Users of plugins should be able to:

• register on app create (zephyr.CreateApp(anchor el, pluginstruct))
• access plugins in components (something like component.App.Plugins.xxx)