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

vendor/github.com/gomarkdown/markdown/parser/matter.go

@@ -0,0 +1,36 @@
+package parser
+
+import (
+	"bytes"
+
+	"github.com/gomarkdown/markdown/ast"
+)
+
+func (p *Parser) documentMatter(data []byte) int {
+	if data[0] != '{' {
+		return 0
+	}
+
+	consumed := 0
+	matter := ast.DocumentMatterNone
+	if bytes.HasPrefix(data, []byte("{frontmatter}")) {
+		consumed = len("{frontmatter}")
+		matter = ast.DocumentMatterFront
+	}
+	if bytes.HasPrefix(data, []byte("{mainmatter}")) {
+		consumed = len("{mainmatter}")
+		matter = ast.DocumentMatterMain
+	}
+	if bytes.HasPrefix(data, []byte("{backmatter}")) {
+		consumed = len("{backmatter}")
+		matter = ast.DocumentMatterBack
+	}
+	if consumed == 0 {
+		return 0
+	}
+	node := &ast.DocumentMatter{Matter: matter}
+	p.addBlock(node)
+	p.finalize(node)
+
+	return consumed
+}