Change rule syntax

lib/imbecile/grammar.rb

@@ -49,7 +49,7 @@ module Imbecile
       elsif input.slice!(/\Adrop\s+(\S+)\n/)
         pattern = $1
         @tokens << Token.new(nil, pattern, @tokens.size)
-      elsif input.slice!(/\Arule\s+(\S+)\s+\[(.*?)\] <<\n(.*?)^>>\n/m)
+      elsif input.slice!(/\A(\S+)\s*:\s*\[(.*?)\] <<\n(.*?)^>>\n/m)
         rule_name, rule, code = $1, $2, $3
         @rules << Rule.new(rule_name, rule, code)
       else