Reworked IChannel.IsNsfw to support the new API flag (#771)

IChannel.IsNsfw will now return false when being used on any channel
that is not an ITextChannel. When being used on an ITextChannel, this
will now account for the API flag, and fall back to the channel name.

(this is gross design, thanks discord)

src/Discord.Net.Rest/API/Common/Channel.cs

@@ -29,6 +29,8 @@ namespace Discord.API
         public Optional<string> Topic { get; set; }
         [JsonProperty("last_pin_timestamp")]
         public Optional<DateTimeOffset?> LastPinTimestamp { get; set; }
+        [JsonProperty("nsfw")]
+        public Optional<bool> Nsfw { get; set; }
 
         //VoiceChannel
         [JsonProperty("bitrate")]