[ALOY-342] Allow 'alloy generate' to create platform-specific files

You should be able to create platform-specific views, styles, and controllers using the alloy generate command. Because platform-specific generators only make sense for views, controllers, and styles, only the alloy generate controller and alloy generate view commands will make use of the new *--platform* option for the CLI. If you attempt to use the *--platform* option with other generators, it will be ignored and a warning will be issued.

example 1

alloy generate controller myname --platform ios

The above would generate the following files: * - app/views/ios/myname.xml * - app/controllers/ios/myname.js * - app/styles/ios/myname.tss

example 2

alloy generate view path/to/myname --platform android

The above would generate the following files * - app/views/android/path/to/myname.xml * - app/styles/android/path/to/myname.tss