Add caching compatibility

[roykho]

Currently this plugin works well if there are no caching of any kind involved. However many sites have some sort of cache which renders this plugin useless because it could potentially serve up cached versions of a product page to a user that is not set in the rules.

To remedy this, my proposal would be to either look into the session cookie and find the viewer's location and serve up the content based on that or perhaps add a parameter to the URL to denote what I am trying to show.