Use more idiomatic form for set of strings.

This is a better style for a set, since each value can only be present
or absent.

With bool as value type, each value may be absent, or true or false. It
also uses slightly more memory.

block.go

@@ -456,7 +456,7 @@ func (p *parser) htmlFindTag(data []byte) (string, bool) {
 		i++
 	}
 	key := string(data[:i])
-	if blockTags[key] {
+	if _, ok := blockTags[key]; ok {
 		return key, true
 	}
 	return "", false

markdown.go

@@ -102,49 +102,49 @@ const (
 	TAB_SIZE_EIGHT   = 8
 )
 
-// These are the tags that are recognized as HTML block tags.
+// blockTags is a set of tags that are recognized as HTML block tags.
 // Any of these can be included in markdown text without special escaping.
-var blockTags = map[string]bool{
-	"blockquote": true,
-	"del":        true,
-	"div":        true,
-	"dl":         true,
-	"fieldset":   true,
-	"form":       true,
-	"h1":         true,
-	"h2":         true,
-	"h3":         true,
-	"h4":         true,
-	"h5":         true,
-	"h6":         true,
-	"iframe":     true,
-	"ins":        true,
-	"math":       true,
-	"noscript":   true,
-	"ol":         true,
-	"pre":        true,
-	"p":          true,
-	"script":     true,
-	"style":      true,
-	"table":      true,
-	"ul":         true,
+var blockTags = map[string]struct{}{
+	"blockquote": struct{}{},
+	"del":        struct{}{},
+	"div":        struct{}{},
+	"dl":         struct{}{},
+	"fieldset":   struct{}{},
+	"form":       struct{}{},
+	"h1":         struct{}{},
+	"h2":         struct{}{},
+	"h3":         struct{}{},
+	"h4":         struct{}{},
+	"h5":         struct{}{},
+	"h6":         struct{}{},
+	"iframe":     struct{}{},
+	"ins":        struct{}{},
+	"math":       struct{}{},
+	"noscript":   struct{}{},
+	"ol":         struct{}{},
+	"pre":        struct{}{},
+	"p":          struct{}{},
+	"script":     struct{}{},
+	"style":      struct{}{},
+	"table":      struct{}{},
+	"ul":         struct{}{},
 
 	// HTML5
-	"address":    true,
-	"article":    true,
-	"aside":      true,
-	"canvas":     true,
-	"figcaption": true,
-	"figure":     true,
-	"footer":     true,
-	"header":     true,
-	"hgroup":     true,
-	"main":       true,
-	"nav":        true,
-	"output":     true,
-	"progress":   true,
-	"section":    true,
-	"video":      true,
+	"address":    struct{}{},
+	"article":    struct{}{},
+	"aside":      struct{}{},
+	"canvas":     struct{}{},
+	"figcaption": struct{}{},
+	"figure":     struct{}{},
+	"footer":     struct{}{},
+	"header":     struct{}{},
+	"hgroup":     struct{}{},
+	"main":       struct{}{},
+	"nav":        struct{}{},
+	"output":     struct{}{},
+	"progress":   struct{}{},
+	"section":    struct{}{},
+	"video":      struct{}{},
 }
 
 // Renderer is the rendering interface.