V3 scheduler/custom js scheduling: include elapsed_seconds for learning cards

@dae: Could you include the seconds that have passed since the last time I rated a card in learning - at the moment I only get remaining_steps and scheduled_secs, see here.

If I knew the seconds passed I could modify the next interval based on the actual time that has passed so that if I forgot to do the learning in time and still remember a card I could skip steps and graduate it right away etc. For cards in review I already have elapsed_days.

Thanks for including a powerful interface that also works on mobile.

Seems reasonable, will look into it.