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.rb

@@ -48,6 +48,7 @@ end
 require 'liquid/drop'
 require 'liquid/extensions'
 require 'liquid/errors'
+require 'liquid/interrupts'
 require 'liquid/strainer'
 require 'liquid/context'
 require 'liquid/tag'