support time.Duration for time flags

[timkuijsten]

$ ./filter-spfgreylist -h
Usage of ./filter-spfgreylist:
  -greyexp duration
        duration before greylist attempts expire (default 4h0m0s)
  -passtime duration
        duration before retries are accounted (default 5m0s)
  -whiteexp duration
        duration before whitelists expire (default 720h0m0s)
  -wl-domain string
        filename containing a list of sender domains to whitelist, one per line
  -wl-ip string
        filename containing a list of IP addresses to whitelist, one per line

[timkuijsten]

It should be noted that it does break backwards compatibility because an "s" suffix is required if seconds are specified.

[poolpOrg]

I really like that 👍🏻

[poolpOrg]

thanks !

[timkuijsten]

thank you! :)