Add BossBar Support

[Pawelusze]

I suggest adding bossbar support as a notifier for how much time is left until the end of combat.
The bossbar should function as a progress bar less remaining time means less progress shown.

You would configure it in this section:

combatNotification:
actionbar: 'Combat ends in: {TIME}'
bossbar: 'Combat ends in: {TIME}'

[P1otrulla]

It's possible to add that, but we'd need to rework multification so it supports updating the boss bar. You could do it now, but it would require a lot of hacks to make it work properly — and since it's our library, we can simply update multification to handle these boss bars.