added interrupt class for continue/break statements

When a continue or break statement is executed it pushes an interrupt to a
stack in context. If any non-handled interrupts are present blocks will cease
to execute. The for loop can handle the most recent interrupt in the stack.

lib/liquid/errors.rb

@@ -8,9 +8,4 @@ module Liquid
   class StandardError < Error; end
   class SyntaxError < Error; end
   class StackLevelError < Error; end
-
-
-  class Interrupt < Error; end
-  class BreakInterrupt < Interrupt; end
-  class ContinueInterrupt < Interrupt; end
 end