api: add slow mode

This adds the following property to ITextChannel
- SlowMode (int)

The SlowMode field defines the time in seconds users must wait between
sending messages; if zero, slow-mode is disabled.

Bots are not affected by slow-mode, and as such, no changes need to be
made to the REST client's internal ratelimiting. This is strictly a
read/write cosmetic property for bots.

src/Discord.Net.Core/Entities/Channels/ITextChannel.cs

@@ -13,6 +13,9 @@ namespace Discord
         /// <summary> Gets the current topic for this text channel. </summary>
         string Topic { get; }
 
+        ///<summary> Gets the current slow-mode delay for this channel. 0 if disabled. </summary>
+        int SlowMode { get; }
+
         /// <summary> Bulk deletes multiple messages. </summary>
         Task DeleteMessagesAsync(IEnumerable<IMessage> messages, RequestOptions options = null);
         /// <summary> Bulk deletes multiple messages. </summary>

src/Discord.Net.Core/Entities/Channels/TextChannelProperties.cs

@@ -1,4 +1,6 @@
-namespace Discord
+using System;
+
+namespace Discord
 {
     /// <inheritdoc />
     public class TextChannelProperties : GuildChannelProperties
@@ -11,5 +13,15 @@
         /// Should this channel be flagged as NSFW?
         /// </summary>
         public Optional<bool> IsNsfw { get; set; }
+        /// <summary>
+        /// What the slow-mode ratelimit for this channel should be set to; 0 will disable slow-mode.
+        /// </summary>
+        /// <remarks>
+        /// This value must fall within [0, 120]
+        /// 
+        /// Users with <see cref="ChannelPermission.ManageMessages"/> will be exempt from slow-mode.
+        /// </remarks>
+        /// <exception cref="ArgumentOutOfRangeException">Throws ArgummentOutOfRange if the value does not fall within [0, 120]</exception>
+        public Optional<int> SlowMode { get; set; }
     }
 }