Update editor content on URL hash code changes

[rbardini]

I'm the maintainer of Storybook Playroom Addon, which basically serializes the JSX of rendered stories, then renders Playroom in an iframe with the encoded string in the URL. However, since hash changes don't trigger UI updates, I have to recreate the iframe and completely reload Playroom whenever the value changes (e.g. on story change), a very expensive and slow operation.

It would be useful to update the editor content (i.e. the code, and therefore the preview) on URL hash code changes, allowing instrumentation of Playroom's UI by external tools.

Relates to rbardini/storybook-addon-playroom#2.