Add localization support

[Romejanic]

Add support for localized command and option names. Will also need to rework the choice labels to account for localizations. Possible solutions I'm thinking of include:

• Adding a name_localizations/description_localizations property to commands
• Using a separate locales.json file to contain localization data

e.g. (first proposal)

{
  "command": {
    "description": "My test command",
    "name_localizations": {
      "fr": "commande"
    },
    "description_localizations": {
      "fr": "Ma commande de test"
    }
  }
}

e.g. (second proposal)
commands.json

{
  "command": {
    "description": "My test command"
  }
}

locale.json

{
  "command": {
    "names": {
      "fr": "commande"
    },
    "descriptions": {
      "fr": "My test command"
    }
  }
}

[Romejanic]

To be included in Slasher v2: #21