Wrong regex in StringExtractor feature type

In `<project_root>/src/thrember/features.py`, line 200 - 201:

```py
"mac_addr": re.compile("\\b(?:[0-9A-Fa-f]{2}[:-]){5}(?:[0-9A-Fa-f]{2})\\b"),
"email_addr": re.compile("\\b(?:[0-9A-Fa-f]{2}[:-]){5}(?:[0-9A-Fa-f]{2})\\b"),
```

The regexes for MAC addresses and Email addresses are the same!

I guess this is worth knowing even though the model quality has been good so far.
