WIP - Number to words localisation improvements

I'm trying to improve experience and encourage users to localise ToWords/ToOrdinalWords extension methods.

I've introduced `INumberToWordsConverter`:
```csharp
public interface INumberToWordsConverter
{
    string Convert(int number);
    string ConvertToOrdinal(int number);
}
```

Also I want to clarify how shall behave untranslated converter? Shall it throw `NotSupportedException` or just return `number.ToString()`?