Provide the ability to update the MAC vendor lookup listings

Addresses #6 by adding the ability to manually update the mac_vendor.txt file from a remote URL or a local file (Wireshark manuf).

For instance, pull directly from the Wireshark repo:

```bash
python includes/mac_vendor.py -u
```

Or, use a local copy if so desired:

```bash
python includes/mac_vendor.py -u -f "{path_to_manuf_file}"
```

The addition is implemented as a class method which makes it possible to add to plugin logic if required to trigger auto-update on init etc.