String visitor

[airbreather]

Desired usage:

public sealed class MyVisitor : CsvUTF8StringReaderVisitorBase
{
    public MyVisitor(int maxFieldLength, DecoderFallback decoderFallback)
        : base(maxFieldLength, decoderFallback)
    {
    }

    public override void VisitField(string field)
    {
        // do stuff with this field
    }

    public override void VisitEndOfRecord()
    {
        // same as the base version
    }
}

[airbreather]

I think we can skip the Encoding like #5 did... updating.