Feature/function translations

[tomasfabian]

Added support for custom function translations (invocations):

  private static class F
  {
    [KSqlFunction]
    public static double Abs(double input) => throw new NotSupportedException();

    [KSqlFunction(FunctionName = "Abs")]
    public static double MyAbs(double input) => throw new NotSupportedException();
  }

Expression<Func<Tweet, double>> expression = c => F.Abs(c.Amount);