context/sessions -- set the deck you want to add to/edit/learn from so you don't have to name it every time

so something like

```bash
$ drill-srs add-card algos -q 'question' -a 'answer'

$ drill-srs add-card -q 'question' -a 'answer'
Okay, I added the card 'question' to the most recently used deck "deck".

$ drill-srs set-deck deck2
'deck2' is the current deck now.

$ drill-srs study
Studying 'deck2'.
15 cards to study. After seeing a card, hit enter.
```