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/block.rb

@@ -89,13 +89,27 @@ module Liquid
     end
 
     def render_all(list, context)
-      list.collect do |token|
+      output = []
+      list.each do |token|
+        # Break out if we have any unhanded interrupts.
+        break if context.has_interrupt?
+
         begin
-          token.respond_to?(:render) ? token.render(context) : token
+          # If we get an Interrupt that means the block must stop processing. An
+          # Interrupt is any command that stops block execution such as {% break %} 
+          # or {% continue %}
+          if token.is_a? Continue or token.is_a? Break
+            context.push_interrupt(token.interrupt)
+            break
+          end
+
+          output << (token.respond_to?(:render) ? token.render(context) : token)
         rescue ::StandardError => e
-          context.handle_error(e)
+          output << (context.handle_error(e))
         end
-      end.join
+      end
+
+      output.join
     end
   end
 end