API changes to make it nicer

These are incompatible, tho.

* Theme is now a property of the formatter instead
  of passing it arounf
* get_style_defs is now style_defs

src/formatter.cr

@@ -9,12 +9,15 @@ module Tartrazine
   # This is the base class for all formatters.
   abstract class Formatter
     property name : String = ""
+    property theme : Theme = Tartrazine.theme("default-dark")
 
-    def format(text : String, lexer : Lexer, theme : Theme) : String
+    # Format the text using the given lexer.
+    def format(text : String, lexer : Lexer) : String
       raise Exception.new("Not implemented")
     end
 
-    def get_style_defs(theme : Theme) : String
+    # Return the styles, if the formatter supports it.
+    def style_defs : String
       raise Exception.new("Not implemented")
     end
   end