Use StringComparison or IComparer<string>

• Changed the API to use more general StringComparison enum or the more general IComparer<string> (instead of StringComparer). Note that using StringComparison instead of IComparer<string>/StringComparer should have better performance as there's no need to allocate substrings.
• Made NaturalSortComparer public, so you can construct it directly without an extension method.
• Changed the extension class name (possibly a breaking change if you're not calling it via the extension method).