Log aggregation/tagging from workers through supervisor

The `--log` option no longer depends on `--detach` and now supports basic template support via %w for worker id and %p for process ID, as in `--log myapp-%w.%p.log`.

The supervisor will close/open the log file(s) when it receives `SIGUSR2`, allowing for log rotation via logrotated or similar.

All log lines are prefixed with <timestamp> pid:%p worker:%w using strongloop/strong-log-transformer#1.

I'd like to add flags for controlling JSON formatting and line prefixes in a future PR if possible so that this one doesn't drag out too long. Syslog support makes more sense as a separate PR after this one.

R=@sam-github
AFAICT, fs write streams will close their fd on end, so should not leak.