Make formatter a bit more convenient

src/formatters/html.cr

@@ -58,7 +58,8 @@ module Tartrazine
       "<span #{line_id} #{line_class} style=\"user-select: none;\">#{line_label} </span>"
     end
 
-    def format_text(text : String, lexer : Lexer, outp : IO) : Nil
+    def format_text(text : String, lexer : Lexer, io : IO?) : String?
+      outp = io.nil? ? String::Builder.new("") : io
       tokenizer = Tokenizer.new(lexer, text)
       i = 0
       if surrounding_pre?
@@ -75,6 +76,7 @@ module Tartrazine
         end
       end
       outp << "</code></pre>"
+      return outp.to_s if io.nil?
     end
 
     # ameba:disable Metrics/CyclomaticComplexity