Support specifying iOS targets with -t or --targets options.

[beata]

Hi, I added a new CLI option for assigning resources to targets.

Example:

react-native-asset --targets TargetA TargetB

[gewfy]

@unimonkiez This would be a great feature as we have three targets (dev, staging, production) and need to go into xcode after running react-native-asset to manually add all targets for the files added. Let me know if I can help out here somehow.

[unimonkiez]

Not sure what it does exactly, @gewfy you can take this PR and create a new one with documentation changes included? or @beata if he still wants to make those additions