We need a simple chrome extension that does the following:
- this has to be a chrome extension (with a default_popup), not a packaged app!
- integrates `chrome.identity` to do oauth2 flow against https://api.twitch.tv
- store access token in localStorage
- if the user is not logged in, do the auth flow to log him in
- provide the button to log him out
- if the user is logger in, display his twitch username in the default_popup.