Proposal: keep non-native translations external

This plugin stores all the translations inside the app binaries, which can be a problem if we're trying to add new locales or fix typos via appsync or trying to get theses translations from the internet.

My proposal is the following:

```
{
"native": {
"app.name": "app name",
"ios.plist.....": "other stuff"
},
"mykey": "my translation"
}
```

These translations are stored in a json inside the app directory. Optionally, we could register our own json. For example: store your translations on the web. When the user opens the app, lazily download them (this is what Facebook does. When you open the app for the first time it stops on a screen saying "please wait while we configure ").

Native translations are still built into the app and can still be accessed by `native.app.name`, but they can't be updated by appsync or json download.

Start by tracing how the plugin currently stores and loads translations, including the native translations and the proposed JSON sources. Review how AppSync and optional remote downloads would fit into that flow. Done means agreeing on the design and documenting the behavior for bundled native translations, external translations, and lazy loading.

Written by the indexing model from the issue text.