Global configuration option to select stored languages

[Joxit]

The configuration is stored in pelias-config and is backward compatible.

Select your stored languages

{
  "imports": {
    "langs": ["en", "fr", "it", "nl"]
  }
}

Select your stored languages and override your default language

{
  "imports": {
    "langs": {
      "keep": ["en", "fr", "it", "nl"],
      "default": "fr"
  }
}

related: pelias/api#1505 (comment)
closes: pelias/pelias#867