Return dispatch

[zaceno]

This PR does two things:

• It makes app() return the internal dispatch so it could be used by external utilities, for e g dev tooling
• It makes it so if you ever dispatch with no arguments (wether with the returned function, or in an effect, or return void from an action) it will stop the app

Stopping an app means:

• future calls to dispatch do nothing.
• all possible active subscriptions are stopped
• the DOM is not touched. It remains exactly as it was before the app was stopped.