Management of script references about tune HOT 2 OPEN

In my oppinion it would be nice to add references by some script "meta-tag", analyzed and removed before compilation. Something like;

#r "System.Numerics.Vectors"

using System;

namespace Samples
{
    public class Echoer
    {
        public string Write(string message)
        {
            return message;
        }
    }
}

from tune.

As I described in PR I've put //#r tag instead of #r. Comment sign allow me to use Roslyn Walker. Basicly im looking for all comments and those which contains #r are parsed to script.

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