Feature/allow to extend old locale with custom keys

When using this module with custom locales.
We've faced an issue when service will throw an error `cannot assign property X to undefined`, that was caused by some complex value that was present in new locale and was not in the old one.
Like:

```
old locale
{
"NUMBER_FORMATS": {
    "CURRENCY_SYM": "$",
    "DECIMAL_SEP": ".",
    "GROUP_SEP": ","
  },
  "id": "en-us",
}

new locale
{
"NUMBER_FORMATS": {
    "CURRENCY_SYM": "₴",
    "DECIMAL_SEP": ",",
    "GROUP_SEP": " "
  },
  "TEMPERATURE_FORMATS": {
    "TEMPERATURE_SYM": "°C",
    "TEMPERATURE_NAME": "Цельсія"
  }
  "id": "ua",
}

```

Here `TEMPERATURE_SYM` and `TEMPERATURE_NAME` will throw this error, because `TEMPERATURE_FORMATS` does not exist.

Having empty placeholder for `TEMPERATURE_FORMATS` solves this issue.