Add card 'Ravenous Rats'

This commit also adds a helper function for asking the player to choose a card from a set of cards. This function should be general enough to be used also for situations like `Take the top 3 cards from your library, choose a card, and put it in your hand. ...`

I'm not sure whether the type of `askChooseCard` shouldn't rather be: 
```haskell
askChooseCard :: PlayerRef -> [(SomeObjectRef, a)] -> Magic (SomeObjectRef, a)
```
(ie. always returning the full choice). But this can be changed if we see the need upon implementing further choice cards.