Added an option to add nonce security to the add and remove

[eliotfallon213]

I worked on a site that was "pen" tested recently and to cut a long story short - it was pulled up on not using nonces. It was using this plugin so I have altered it to perform a check on it.

Obviously if a site is cached you won't want to be using this option. I'm fully aware this is a minor issue and wanting to use this feature will be an edge case, but I'd written the code so thought I'd better pay it back as it's a great plugin.

[maxfenton]

Still think this is a good idea.