Show intermediate proposition truth values

When generating a truth table, it will be nice (and educational!) to also show the truth values of sub-propositions. For example, in:

```
~p ≡ (q ∨ r)
```

It would be nice to also show table columns for `~p` and `q ∨ r` in addition to the already displayed atomic variables `p`, `q` and `r` and full sentence. 
