Add auto-update script for Chrome

Follow the instructions in the `README` and this snippet will auto-update itself and any other snippets in the `FrogTrade/fos-snippets` repository.

GitHub credentials can optionally be specified in the form of `window.credentials = ['username', 'password'];` if the unauthenticated API requests hit the rate-limit.

The only tricky bit is opening Chrome DevTools on itself in order for it to work correctly, but there's an alert popup with instructions if you try to run it incorrectly.