Add a custom platform? #327

The web portion of my app has an emulator view (just a phone that content can be passed to).

The ios / android prefixes obviously don't work since it's web. md/sm prefixes also don't work since it's checking the browser dimensions not the emulator's dimensions.

Is there a way I could add 2 custom prefixes?

preview - This will be active if within a PhoneProvider.
mobile - This will be ios, android or preview.