* Clone() method in Localizer component allows to clone it just like LocalizationMiddleware does.
* RavenClientInterface contains all *raven.Client method, which allows external use without type casting.
* Tests and godoc fix for Go 1.15.
* proposal with context localizer (engine.GetLocalizedMessage shouldn't be used for localizing for individual requests data)
* normal test for SetLocale
* use sync.Map
* method to preload languages into localizer
* DefaultLanguages variable
* PreloadLanguages field in core.Engine
* don't copy sync.Map
* patch for goroutine safety usage of Localizer
* fix for engine constructor
* additional test for localizer middleware, which is more close to real environment
* filter language.Und
* prevent root placeholder tag usage
* remove reduntant bundle storage, deprecated (*core.Localizer).FetchLanguage()
