feat: Whitelist URL feature.

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.

configuration.default.ini

@@ -183,6 +183,9 @@ user_unban = userunban
 url_ban = urlban
 url_ban_list = urlbanlist
 url_unban = urlunban
+url_whitelist = urlwhitelist, urlw
+url_unwhitelist = urlunwhitelist, urlunw
+url_whitelist_list = urlwhitelistlist, urlwls
 
 ducking = duck
 ducking_threshold = duckthres