Add round-up offset feature to bb_now

[tomocafe]

Use ^ to denote rounding up to the nearest, e.g. +0d^ means the upcoming 00:00, +1h^ at 06:30 means 08:00, -1h^ at 06:30 means 06:00, etc.

^d, ^h can be shorthand for +0d^, +0h^ (etc.)