Add optional 'id' member to manifest

According to [W3C spec](https://w3c.github.io/manifest/#id-member):

> The `id` member is a string that represents the identity for the application.

Additional information by [Google's chrome for developers](https://developer.chrome.com/docs/capabilities/pwa-manifest-id):

> The `id` property represents the identity of the PWA to the browser.