refactor: Removed the AssetImage inheritance from AssetGenImage

[wasabeef]

What does this change?

• [BREAKING CHANGE] Removed the AssetImage inheritance from AssetGenImage. This will solve the following.
  - Extending package information to asset types other than AssetGenImage. #162
  - Added 'gen_for_package:true/false' param to support asset generation for a package #213

  And this requires the following changes:

  // Before
  Widget build(BuildContext context) {
    return Image(image: Assets.images.chip);  // Can't use this.
  }
  // After
  Widget build(BuildContext context) {
    return Assets.images.chip.image();
  }

• Added example_resource package for how to use another package resources from an app.

# file: example_resources/pubspec.yaml
# ... 
flutter_gen:
  # ...
  assets:
    enabled: true
    package_parameter_enabled: true

#...