Added a TAG parameter to prepend to each log line

Because of my Java background, I usually prepend log messages with a TAG to filter out the log based on cluster, using a command like:
```
tail -f "/Users/`whoami`/Library/Logs/iOS Simulator/${VERSION}/system.log" | grep "${TAG}"
```

I'd have used '->', as tag to filter, but adding a custom one, can add more flexibility.

Moreover, I refactored a bit