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

vendor/github.com/gomarkdown/markdown/ast/attribute.go

@@ -0,0 +1,10 @@
+package ast
+
+// An attribute can be attached to block elements. They are specified as
+// {#id .classs key="value"} where quotes for values are mandatory, multiple
+// key/value pairs are separated by whitespace.
+type Attribute struct {
+	ID      []byte
+	Classes [][]byte
+	Attrs   map[string][]byte
+}