Budget feature

I think about a budget feature where the user can track how much money was
spent on the hobby

* Fully optional
* living under `serra budget list`
* compare what were the spendings and compare to what cards are worth (am i
    really ~investing~ here?)

I imagine something like

```
Cards worth: 250 $ (currently)
Spent: 273 $
Overall: -23 $
```

