gfm tests:

```
% pandoc -f gfm -t native
| Fruit | Price |
| ----- | ----: |
| apple | 0.13  |
| orange|1.12|
^D
[ Table
  ( "", [], [] )
  ( Caption Nothing [] )
  [ ( AlignDefault, ColWidthDefault ), ( AlignRight, ColWidthDefault ) ]
  ( TableHead
    ( "", [], [] )
    [ Row
      ( "", [], [] )
      [ Cell
        ( "", [], [] ) AlignDefault
        ( RowSpan 1 )
        ( ColSpan 1 )
        [ Plain [ Str "Fruit" ] ]
      , Cell
        ( "", [], [] ) AlignDefault
        ( RowSpan 1 )
        ( ColSpan 1 )
        [ Plain [ Str "Price" ] ]
      ]
    ]
  )
  [ TableBody
    ( "", [], [] )
    ( RowHeadColumns 0 ) []
    [ Row
      ( "", [], [] )
      [ Cell
        ( "", [], [] ) AlignDefault
        ( RowSpan 1 )
        ( ColSpan 1 )
        [ Plain [ Str "apple" ] ]
      , Cell
        ( "", [], [] ) AlignDefault
        ( RowSpan 1 )
        ( ColSpan 1 )
        [ Plain [ Str "0.13" ] ]
      ]
    , Row
      ( "", [], [] )
      [ Cell
        ( "", [], [] ) AlignDefault
        ( RowSpan 1 )
        ( ColSpan 1 )
        [ Plain [ Str "orange" ] ]
      , Cell
        ( "", [], [] ) AlignDefault
        ( RowSpan 1 )
        ( ColSpan 1 )
        [ Plain [ Str "1.12" ] ]
      ]
    ]
  ]
  ( TableFoot ( "", [], [] ) [] )
]
```

```
% pandoc -f gfm -t native
~~stricken out~~
^D
[ Para [ Strikeout [ Str "stricken", Space, Str "out" ] ] ]
```

```
% pandoc -f gfm -t native
# Header
## Header
# -foo-bar_baz
^D
[ Header 1
  ( "header", [], [] )
  [ Str "Header" ]
, Header 2
  ( "header-1", [], [] )
  [ Str "Header" ]
, Header 1
  ( "-foo-bar_baz", [], [] )
  [ Str "-foo-bar_baz" ]
]
```

```
% pandoc -f gfm -t native
My:thumbsup:emoji:heart:
^D
[ Para
  [ Str "My"
  , Span
    ( "", [ "emoji" ], [ ( "data-emoji", "thumbsup" ) ] )
    [ Str "\128077" ]
  , Str "emoji"
  , Span
    ( "", [ "emoji" ], [ ( "data-emoji", "heart" ) ] )
    [ Str "\10084\65039" ]
  ]
]
```

```
% pandoc -f gfm -t native
"hi"
^D
[ Para [ Str "\"hi\"" ] ]
```

```
% pandoc -f gfm+smart -t native
"hi"
^D
[ Para [ Quoted DoubleQuote [ Str "hi" ] ] ]
```

```
% pandoc -t gfm -f native
[Table ("",[],[]) (Caption Nothing
 [Plain [Str "The",Space,Str "caption."]])
 [(AlignDefault,ColWidthDefault)
 ,(AlignRight,ColWidthDefault)]
 (TableHead ("",[],[])
 [Row ("",[],[])
  [Cell ("",[],[]) AlignDefault (RowSpan 1) (ColSpan 1)
   [Plain [Str "Fruit"]]
  ,Cell ("",[],[]) AlignDefault (RowSpan 1) (ColSpan 1)
   [Plain [Str "Price"]]]])
 [(TableBody ("",[],[]) (RowHeadColumns 0)
  []
  [Row ("",[],[])
   [Cell ("",[],[]) AlignDefault (RowSpan 1) (ColSpan 1)
    [Plain [Str "apple"]]
   ,Cell ("",[],[]) AlignDefault (RowSpan 1) (ColSpan 1)
    [Plain [Str "0.13"]]]
  ,Row ("",[],[])
   [Cell ("",[],[]) AlignDefault (RowSpan 1) (ColSpan 1)
    [Plain [Str "orange"]]
   ,Cell ("",[],[]) AlignDefault (RowSpan 1) (ColSpan 1)
    [Plain [Str "1.12"]]]])]
 (TableFoot ("",[],[])
 [])]
^D
| Fruit  | Price |
|--------|------:|
| apple  |  0.13 |
| orange |  1.12 |

The caption.
```

```
% pandoc -f gfm-smart -t gfm+smart
“hi”
^D
"hi"
```

```
% pandoc -f gfm+smart -t gfm-smart
"hi"
^D
“hi”
```

```
% pandoc -f gfm+smart -t gfm+smart
"hi"
^D
"hi"
```

```
% pandoc -f gfm+hard_line_breaks -t native
hi
hi
^D
[ Para [ Str "hi", LineBreak, Str "hi" ] ]
```

```
% pandoc -f gfm -t native
- [ ] foo
- [x] bar
^D
[ BulletList
  [
    [ Plain [ Str "\9744", Space, Str "foo" ] ]
  ,
    [ Plain [ Str "\9746", Space, Str "bar" ] ]
  ]
]
```

```
% pandoc -f gfm-task_lists -t native
- [ ] foo
- [x] bar
^D
[ BulletList
  [
    [ Plain [ Str "[", Space, Str "]", Space, Str "foo" ] ]
  ,
    [ Plain [ Str "[x]", Space, Str "bar" ] ]
  ]
]
```

```
% pandoc -f gfm -t gfm
- [ ] foo
- [x] bar
^D
-   [ ] foo
-   [x] bar
```