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.

lang/zh_CN.json

@@ -61,13 +61,13 @@
         "shortlist_instruction": "使用<i>!sl {indexes}</i>播放列表中的曲目。",
         "start_updating": "开始更新……",
         "stopped": "音乐停止。",
-        "too_long": "<b>{song}</b>超出长度限制！已被移出播放列表。",
+        "too_long": "<b>{song}</b>超出长度限制（{duration} > {max_duration}）！已被移出播放列表。",
         "unable_download": "无法下载<b>{item}</b>。已移出播放列表。",
         "unable_play": "无法播放<b>{item}</b>。已移出播放列表。",
         "unknown_mode": "未知播放模式\"{mode}\"。播放模式应为 <i>one-shot</i>, <i>repeat</i>, <i>random</i>中的一个。",
         "update_successful": "<h2>botamusique v{version} 安装完毕！</h2><hr />\n<h3>更新日志</h3> {changelog} <hr /> 请访问我们的 <a href=\"https://github.com/azlux/botamusique\">github页面</a> 获取更多信息！",
         "url": "URL",
-        "url_ban": "该链接被列入黑名单了！",
+        "url_ban": "链接{url}被列入黑名单了！",
         "url_from_playlist": "URL",
         "url_from_playlist_item": "<a href=\"{url}\"><b>{title}</b></a>，来自播放列表 <a href=\"{playlist_url}\">{playlist}</a>，由<i> {user} </i>添加。",
         "url_item": "<a href=\"{url}\"><b>{title}</b></a>，<i>由</i> {user} <i>添加</i>。",