Support mapping will translate languages

[little-alei]

i18n-ally is very good, but when I use machine translation, for example, I need to use Baidu API to translate the target into traditional characters, and the Chinese traditional Chinese code supported by Baidu is cht, not zh-TW, so the translation does not take effect. So I created this Pr and now we can make it work like this

"i18n-ally.translate.target.map": {
  "zh-HK": "cht",
  "zh-TW": "cht",
}