Feature Request: Add --monitor mode instead of using watch

[Massimo-B]

Instead of using watch -n1 -cd genlop -c it could be interesting to add a --monitor mode doing the same thing.
I guess that genlop -c always needs to parse the emerge.log, being an expensive call every 1s using watch.
Adding a --monitor mode could improve the calculation by only parsing once in a while.

This would make genlop be a real emerge monitor, adding ETA and progress calculations that emerge itself doesn't provide.