On uninstall, prompt user to delete data

`uninstall.php` will run when the user deletes the plugin from the WordPress Plugins page. Prompt the user to delete the custom post type `library` entries with a message, "Would you like to delete all data associated with this plugin?". If the user says, "Yes", then delete the data. If the user says, "No", then proceed to delete the plugin without altering the data.
