feat: add --target option

[szymonrybczak]

Summary:

Closes #1922

With this option we can specify what target will be used to run/build iOS app.

Test Plan:

1. Clone the repository and do all the required steps from the Contributing guide
2. Inside iOS project duplicate the Xcode target and its build scheme.
3. Run these commands:
   If target doesn't exist should select first available one:

node /path/to/react-native-cli/packages/cli/build/bin.js run-ios --target "someRandomTargetName" 

Should select target "second" for scheme "first":

node /path/to/react-native-cli/packages/cli/build/bin.js run-ios --scheme "first" --target "second" 

[adamTrz]

Lgtm :)