Add option to make destination parameter optional

It would be nice to have the destination parameter be optional. For example some custom contextual links may want to lead to forms that you don't want to immediately redirect to the destination after saving (e.g. when using the "Add Content" button on nodequeues you want the form to reload so that you can sort the item you just added).

The attached patch adds a settings whose default value is enabled (for backwards compatibility).

Thanks!