Add option to IgnoreHeaderUnderscores

[kimsey0]

Implement a IgnoreHeaderUnderscores configuration setting that mirrors IgnoreHeaderWhiteSpace.

[JoshClose]

I took a different approach to this by adding Func<string, string> Configuration.PrepareHeaderForMatch { get; set; }. This allows the user to do whatever they need to do. There were too many different config values for how to match against headers.