goldmark用Go语言编写的markdown解析器

            
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            +-------- parser.Parser ---------------------------
            | 1. Parse block elements into AST
            |   1. If a parsed block is a paragraph, apply 
            |      ast.ParagraphTransformer
            | 2. Traverse AST and parse blocks.
            |   1. Process delimiters(emphasis) at the end of
            |      block parsing
            | 3. Apply parser.ASTTransformers to AST
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            +------- renderer.Renderer ------------------------
            | 1. Traverse AST and apply renderer.NodeRenderer
            |    corespond to the node type

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