Session04 - trigrams.py finished

Actually, the problem here is twofold.  First, calling `trigramtable.keys()` creates a list out of all the keys in the `trigramtable` dict.  Second, the lookup for k in this new list takes place in O(n) time.  It would be immensely faster simply to take advantage of direct dict membership:

```python
if k in trigramtable:
    trigramtable[k].append(word)
```