Describe the Feature
This is similar to what has been brought up on #142, basically add a simpler way to specify which device/simulator you want to run your app on.
Possible Implementations
react-native run-ios --device="iPhone X" # will pick up the first iPhone X
react-native run-ios --device="iPhone X 12.1"
react-native run-ios --list-devices # will list devices for iOS
react-native --list-devices # list connected devices, maybe for both platforms