feat: create combination_loader

[imfelixng]

Problem: We may need to combine many loaders from many sources and get a final result. So I created combination_loader to help combine a list of loaders like RootBundleAssetLoader, HttpAssetLoader, and so on.

To apply it, we use it simply by putting it as a loader in EasyLocalization widget.

...
EasyLocalization(
  child: MyApp(),
  supportedLocales: [
    Locale('en', 'US'),
    Locale('ar', 'DZ'),
  ],
  assetLoader: CombinationLoader(
    loaders: [
      RootBundleAssetLoader(...),
      HttpAssetLoader(...),
    ]
  )
)
...

[lambui09]

LGTM.