[JS] Add 'react-native log'

[mkonicek]

Reading logs with React Native should be super easy, right now it's not. You have to:

• Run adb logcat on Android
• Use Xcode, Nuclide etc. on iOS. See http://stackoverflow.com/questions/30115372/how-to-do-logging-in-react-native

We should have a command 'react-native log' that prints logs for a running Android emulator / iOS simulator or connected device. For that we'll need to detect whether an Android or iOS simulator is running or a device connected (e.g. using adb devices or something similar on iOS). It seems reasonable to:

• Start simple and first add two separate commands: react-native log-android, react-native log-ios
• Later add react-native log that detects what kind of devices are connected and calls either log-android or log-ios.

The command should be implemented as part of the CLI. To get started, see https://github.com/facebook/react-native/blob/master/local-cli/cliEntry.js

[grabbou]

In terms of the command itself, I'd suggest this should be done as an rnpm-plugin so that we can merge it easily and it won't produce any conflicts with our PR?

[mkonicek]

react-native log-ios and log-android implemented in #8352 🎉

[farwayer]

Nice! But what is about using command with several connected devices? Ideally it will be nice to have a possibility to specify device id for command. Something like react-native log <IOS-OR-ANDROID-DEVICE-ID> or simply react-native log if it is only one device connected.

And react-native devices command like adb devices but platform independent.