Tag for the next one in schedule - @next-oncall

[LiorHalfon]

Would be nice to be able to add a tag which will be the next one in the schedule.
For example, let's say the schedule is:

1. personA
2. personB
   And personA is the on-call now, so @current-oncall will be personA, and @next-oncall will be personB.

I think this will be useful when you write to update the next on call about an issue or about things he should be aware of.