Make formatter a bit more convenient

src/formatter.cr

@@ -12,7 +12,7 @@ module Tartrazine
     property theme : Theme = Tartrazine.theme("default-dark")
 
     # Format the text using the given lexer.
-    def format(text : String, lexer : Lexer, io : IO?) : String?
+    def format(text : String, lexer : Lexer, io : IO? = nil) : String?
       raise Exception.new("Not implemented")
     end
 

src/formatters/ansi.cr

@@ -11,7 +11,7 @@ module Tartrazine
       "#{i + 1}".rjust(4).ljust(5)
     end
 
-    def format(text : String, lexer : Lexer, io : IO?) : String?
+    def format(text : String, lexer : Lexer, io : IO? = nil) : String?
       outp = io.nil? ? String::Builder.new("") : io
       tokenizer = Tokenizer.new(lexer, text)
       i = 0

src/formatters/html.cr

@@ -34,7 +34,7 @@ module Tartrazine
                    @weight_of_bold : Int32 = 600)
     end
 
-    def format(text : String, lexer : Lexer, io : IO?) : String?
+    def format(text : String, lexer : Lexer, io : IO? = nil) : String?
       outp = io.nil? ? String::Builder.new("") : io
       pre, post = wrap_standalone
       outp << pre if standalone?

src/formatters/json.cr

@@ -4,7 +4,7 @@ module Tartrazine
   class Json < Formatter
     property name = "json"
 
-    def format(text : String, lexer : Lexer, io : IO?) : String?
+    def format(text : String, lexer : Lexer, io : IO? = nil) : String?
       outp = io.nil? ? String::Builder.new("") : io
       tokenizer = Tokenizer.new(lexer, text)
       outp << Tartrazine::Lexer.collapse_tokens(tokenizer.to_a).to_json