Update markdown parsing library to github.com/gomarkdown/markdown (#944)

vendor/github.com/gomarkdown/markdown/html/doc.go

@@ -0,0 +1,43 @@
+/*
+Package html implements HTML renderer of parsed markdown document.
+
+Configuring and customizing a renderer
+
+A renderer can be configured with multiple options:
+
+	import "github.com/gomarkdown/markdown/html"
+
+	flags := html.CommonFlags | html.CompletePage | html.HrefTargetBlank
+	opts := html.RenderOptions{
+		TItle: "A custom title",
+		Flags: flags,
+	}
+	renderer := html.NewRenderer(opts)
+
+You can also re-use most of the logic and customize rendering of selected nodes
+by providing node render hook.
+This is most useful for rendering nodes that allow for design choices, like
+links or code blocks.
+
+	import (
+		"github.com/gomarkdown/markdown/html"
+		"github.com/gomarkdown/markdown/ast"
+	)
+
+	// a very dummy render hook that will output "code_replacements" instead of
+	// <code>${content}</code> emitted by html.Renderer
+	func renderHookCodeBlock(w io.Writer, node *ast.Node, entering bool) (ast.WalkStatus, bool) {
+		_, ok := node.Data.(*ast.CodeBlockData)
+		if !ok {
+			return ast.GoToNext, false
+		}
+		io.WriteString(w, "code_replacement")
+		return ast.GoToNext, true
+	}
+
+	opts := html.RendererOptions{
+		RenderNodeHook: renderHookCodeBlock,
+	}
+	renderer := html.NewRenderer(opts)
+*/
+package html