Provide option to merge translations

[alexlafroscia]

In order to keep translations small, I've been providing only differences from English in my translation files, and then having all locales fall back to the en-US translation file if nothing else can be found.

It would be cool to allow this to be generated automatically through a config option, like:

yo skatejs:translation fr-CA --fallback fr-FR,en-US

or something similar, to do the following in the Translation helper automatically

import enUS from 'whereever';
import frFR from 'whereever';
import frCA from 'whereever';

const translationMap = new Map();
...
translationMap.set('fr-CA', merge(enUS, frFR, frCA));

where merge is like Object.assign except that it supports deep merging of keys with an object for a value. My personal implementation uses deepmerge, which might be an appropriate initial approach.

• Provide fallback option that allows a locale to fall back to other arbitrary locales
  • Fail if a fallback locale does not exist
• Automatically install deepmerge and set up the merge function the first time only
• When the fallback option is provided, generate the map's value by merging locales instead of simply using the imported file