help message
```
$ topcron -h
topcron 0.1.3
A Cron log parser and viewer.
To see status and duration of cronjobs, please add `-L 15` to EXTRA_OPTS of cron, then restart the
daemon.
USAGE:
topcron [OPTIONS]
OPTIONS:
-0, --ok-filter Display only successfull cronjobs
-1, --ko-filter Display only failed cronjobs (include unknow status)
-f, --file <FILE> File(s) to parse [default: /var/log/syslog]
-F, --filter <FILTER> Filter (user, pid, command...) specific cronjobs [default: ]
-h, --help Print help information
-t, --timestamp-mode Display dates and duration in timestamp format
-T, --truncate-mode Don't truncate command line
-u, --user-filter <USER_FILTER> Filter (user, pid, command...) specific cronjobs [default: ]
-v, --verbose Show debug messages
-V, --version Print version information
```