Fix figure caption not showing in produced docx or HTML document

[ZhongYic00]

Using the latest version, I cannot see caption in docx output, and the figure is made inline with wrong style(Body, should be Captioned Figure).
That's because Image is inline node, while Figure is a block node consists of (FigureCaption,Array<block>).
Pandoc auto-generated AST use such structure, only so can the caption be displayed in docx output (also applies to HTML output in my test).
Below is a piece of auto-gen AST:

Figure
    ( "superfig" , [] , [] )
    (Caption
       Nothing
       [ Plain
           [ Str "caption"
           , Space
           , Str "text\22270\24335\20449\24687"
           ]
       ])
    [ Plain
        [ Image
            ( "" , [] , [] )
            [ Str "caption"
            , Space
            , Str "text\22270\24335\20449\24687"
            ]
            ( "/home/zyc/Pictures/whu-logo.png" , "" )
        ]
    ]

The produced HTML of test.md:

---

This PR bases upon the bump version PR #122

[BrentHuang]

实测通过，谢谢！