Automatic reloaded when stumptown-content files change

[peterbe]

User story:

1. Check out stumptown-renderer and make install, make run-server etc.
2. Start viewing it in the browser. E.g. http://localhost:3000/docs/Web/HTML/Element/video/
3. Open up something like stumptown/content/html/reference/elements/video/docs.md in an editor like vim or vscode.
4. Make some changes and hit Ctrl-S to save the file.
5. For the web page to automatically reload to show these new changes.

This daemon that monitors file changes would need to be something that developers ideally run in the foreground. That way they can see if a typo, for example, breaks the rendering update. So it would be nice to make it all pretty and colorful.
We could also do what create-react-app's webpack dev server does which is to catch errors in the daemon, send them to the browser and instead of reloading the page, show the whole error fullscreen.