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/context.rb

@@ -22,6 +22,8 @@ module Liquid
       @errors         = []
       @rethrow_errors = rethrow_errors
       squash_instance_assigns_with_environments
+
+      @interrupts = []
     end
 
     def strainer
@@ -41,6 +43,21 @@ module Liquid
       end
     end
 
+    # are there any not handled interrupts?
+    def has_interrupt?
+      !@interrupts.empty?
+    end
+
+    # push an interrupt to the stack. this interrupt is considered not handled.
+    def push_interrupt(e)
+      @interrupts.push(e)
+    end
+
+    # pop an interrupt from the stack
+    def pop_interrupt
+      @interrupts.pop
+    end
+
     def handle_error(e)
       errors.push(e)
       raise if @rethrow_errors