feat: locale aliases

[mdreizin]

It would be nice to have ability to map smartling to application locales like repository-connector does:

project.go

func resolveLocale(locales map[string]string, locale string) string {
    if val, ok := locales[locale]; ok {
        return val
    }

    return locale
}

func localPullFilePath(p, locales map[string]string, locale string) string {
    parts := FilenameParts{
        Path:   p,
        Dir:    path.Dir(p),
        Base:   path.Base(p),
        Ext:    path.Ext(p),
        Locale: resolveLocale(locales, locale),
    }
    // ...
}

smartling.yml

PullFilePath: '{{ .Dir }}/{{.Locale}}{{.Ext}}'
Locales:
  en-US: en_US
  ru-RU: ru
  # etc