Give `/eval` superpowers

- [x] Evaluate exchange rates: #77 
```
/eval 0.01 * $BTCUSDT

Output:
495.568
```

- [x] Evaluate stock prices: #71 
```
/eval 100 * $BUKA

Output:
94500
```

- [ ] Unit conversions
```
Spec to be discussed
```

- [ ] Google knowledge graph
```
/eval Person("taylor swift").height

Output:
180
```
