Document BlockStack and make linter happ[y/ier].

ansi/codeblock.go

@@ -10,7 +10,7 @@ import (
 	"github.com/muesli/reflow/indent"
 )
 
-// CodeBlockElements are used to render code blocks.
+// A CodeBlockElement is used to render code blocks.
 type CodeBlockElement struct {
 	Code     string
 	Language string