Adding thousands separator to lag output

Added thousands separator for lag output
1087404
becomes
1,087,404

This commit is debatable because using this output programmatically will make the thousands separator more of an annoyance. 
Could be passed as another cmd line option or just not needed at all. I'm open to suggestions, It might only be useful for the sumPart option
... and then here, just wrap with an if

```
val fmtedLag = if (args.formatLagOutput) NumberFormat.getIntegerInstance().format(lag) else lag
"%-15s\t%-40s\t%-15s\t%-15s\t%-15s".format(head.group, head.topic, offset, log, fmtedLag)
```