In the configuration, `max_duration` can be set to prevent
long song(URL item) being downloaded and added to the
playlist.
This whitelist feature provided a way to override this
duration check: songs being whitelisted will be added to
the playlist no matter how long they are.
Three admin commands are introduced:
- !urlwhitelist (!urlw)
- !urlunwhitelist, (!urlunw)
- !urlwhitelistlist, (!urlwls).
Also, if one song fails due to its length, the bot will
show the length of this song and the max length limit in
the reply message.
Implement #173, #196.
