Results marker can be hidden in block attributes (#3706)

```
% pandoc -f org -t native
#+begin_src r :exports results :colnames yes
   data.frame(Id = 1:3, Desc = rep("La",3))
#+end_src

#+caption: Lalelu.
#+label: tab
#+RESULTS:
| Id | Desc |
|----+------|
|  1 | La   |
|  2 | La   |
|  3 | La   |
^D
[ Table
    ( "tab" , [] , [] )
    (Caption Nothing [ Plain [ Str "Lalelu." ] ])
    [ ( AlignDefault , ColWidthDefault )
    , ( AlignDefault , ColWidthDefault )
    ]
    (TableHead
       ( "" , [] , [] )
       [ Row
           ( "" , [] , [] )
           [ Cell
               ( "" , [] , [] )
               AlignDefault
               (RowSpan 1)
               (ColSpan 1)
               [ Plain [ Str "Id" ] ]
           , Cell
               ( "" , [] , [] )
               AlignDefault
               (RowSpan 1)
               (ColSpan 1)
               [ Plain [ Str "Desc" ] ]
           ]
       ])
    [ TableBody
        ( "" , [] , [] )
        (RowHeadColumns 0)
        []
        [ Row
            ( "" , [] , [] )
            [ Cell
                ( "" , [] , [] )
                AlignDefault
                (RowSpan 1)
                (ColSpan 1)
                [ Plain [ Str "1" ] ]
            , Cell
                ( "" , [] , [] )
                AlignDefault
                (RowSpan 1)
                (ColSpan 1)
                [ Plain [ Str "La" ] ]
            ]
        , Row
            ( "" , [] , [] )
            [ Cell
                ( "" , [] , [] )
                AlignDefault
                (RowSpan 1)
                (ColSpan 1)
                [ Plain [ Str "2" ] ]
            , Cell
                ( "" , [] , [] )
                AlignDefault
                (RowSpan 1)
                (ColSpan 1)
                [ Plain [ Str "La" ] ]
            ]
        , Row
            ( "" , [] , [] )
            [ Cell
                ( "" , [] , [] )
                AlignDefault
                (RowSpan 1)
                (ColSpan 1)
                [ Plain [ Str "3" ] ]
            , Cell
                ( "" , [] , [] )
                AlignDefault
                (RowSpan 1)
                (ColSpan 1)
                [ Plain [ Str "La" ] ]
            ]
        ]
    ]
    (TableFoot ( "" , [] , [] ) [])
]
```

```
% pandoc -f org -t native
#+begin_src R :exports none :colnames yes
   data.frame(Id = 1:2, Desc = rep("La",2))
#+end_src

#+caption: Lalelu.
#+label: tab
#+RESULTS:
| Id | Desc |
|----+------|
|  1 | La   |
|  2 | La   |
^D
[]
```
