Add support for not closeable tabs

[se7kn8]

With these changes, it's possible to have tabs which are not closeable by the user. To make it easier to close them programmatically, the addTab function now returns the created element, which can be later used to remove the tab.

[adamschwartz]

@SE7-KN8 Thanks so much for your contribution. After digging into it, I think I’d prefer to implement it in the reverse: keep the element in the template, but remove it if the option is passed. Would you mind making this change?

[se7kn8]

No problem! I just changed it.