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/interrupts.rb

@@ -0,0 +1,17 @@
+module Liquid
+
+  # An interrupt is any command that breaks processing of a block (ex: a for loop).
+  class Interrupt
+    attr_reader :message
+
+    def initialize(message=nil)
+      @message = message || "interrupt"
+    end
+  end
+
+  # Interrupt that is thrown whenever a {% break %} is called.
+  class BreakInterrupt < Interrupt; end
+
+  # Interrupt that is thrown whenever a {% continue %} is called.
+  class ContinueInterrupt < Interrupt; end
+end