Make formatter a bit more convenient

src/formatters/json.cr

@@ -4,11 +4,16 @@ module Tartrazine
   class Json < Formatter
     property name = "json"
 
-    def format(text : String, lexer : Lexer, io : IO? = nil) : String?
-      outp = io.nil? ? String::Builder.new("") : io
+    def format(text : String, lexer : Lexer) : String
+      outp = String::Builder.new("")
+      format(text, lexer, outp)
+      return outp.to_s
+    end
+
+
+    def format(text : String, lexer : Lexer, io : IO) : Nil
       tokenizer = Tokenizer.new(lexer, text)
-      outp << Tartrazine::Lexer.collapse_tokens(tokenizer.to_a).to_json
-      return outp.to_s if io.nil?
+      io << Tartrazine::Lexer.collapse_tokens(tokenizer.to_a).to_json
     end
   end
 end