Support log KeepMany

Right now you have only `KeepAll` or `KeepOne`, neither are really good IMO.
Usually you want a balance where you keep the last X files, either by restarts or (ideally) by day.

So the new enum could look like:
```rust
enum LogRotation {
  KeepOne
  KeepAll
  KeepMany(usize)
}
```