helpers: Add Blackfriday 'joinLines' extension support (#3574)

See russross/blackfriday#334
"add an extension to handle Chinese (or CJK) newlines"
for more information.

helpers/content.go

@@ -137,6 +137,7 @@ var blackfridayExtensionMap = map[string]int{
 	"autoHeaderIds":          blackfriday.EXTENSION_AUTO_HEADER_IDS,
 	"backslashLineBreak":     blackfriday.EXTENSION_BACKSLASH_LINE_BREAK,
 	"definitionLists":        blackfriday.EXTENSION_DEFINITION_LISTS,
+	"joinLines":              blackfriday.EXTENSION_JOIN_LINES,
 }
 
 var stripHTMLReplacer = strings.NewReplacer("\n", " ", "</p>", "\n", "<br>", "\n", "<br />", "\n")