Allow overriding translations

[rayz1065]

Closes #48

For some use-cases it would be useful to have the ability to override certain translations programmatically, for example by loading multiple translation files before loading the default translations.

This pull request changes the matchBundles function to allow matching multiple bundles for the same locale before falling back to the default bundle.

Within the bot it is then possible to use a piece of code such as:

function loadLocaleData() {
  if (process.env.LOCALE_OVERRIDES) {
    const overrides = process.env.LOCALE_OVERRIDES.split(',');
    overrides.forEach((override: string) => {
      const directory = path.join(__dirname, 'i18n', override);
      console.log('loading locale override', override);
      i18n.loadLocalesDirSync(directory);
    });
  }

  i18n.loadLocalesDirSync(path.join(__dirname, 'i18n'));
}

When paired with a properly implemented warnings handler it allows properly overriding translations.

{
  fluentOptions: {
    warningHandler: (warning) => {
      if (
        warning.type === TranslateWarnings.MISSING_MESSAGE &&
        !warning.bundle.hasMessage('-override')
      ) {
        // ignore warning on override file
        return;
      }

      defaultWarningHandler(console.log)(warning);
    },
  },
}

Note: the above example assumes a message with key '-override' exists on override files.