feature: generate ressources for specific paltform

[danielsogl]

What version of the CLI are you using? Output of ionic --version

3.0.0-beta8

Short description of the problem:

In CLI 2.0 it was possible to generate the icons and splash screen for a specific platform with the platform tag like ionic resources android

In V3 it should also be possible because I use different Icons for Android and iOS. Otherwise, the command always overrides the icons and splash screens of both platforms.

ionic cordova resources android

[imhoffd]

This will be in the next release of the CLI. Thank you!