Add 'html' to EmbedType enum

This resolves #762.

This change adds an 'html' variant to the EmbedType enum.

This change also adds an 'Unknown' variant to the EmbedType enum (at
position -1); this will be used in a later commit to future-proof the
EmbedType enum from any further variants Discord may add.

src/Discord.Net.Core/Entities/Messages/EmbedType.cs

@@ -1,13 +1,15 @@
-namespace Discord
+namespace Discord
 {
     public enum EmbedType
     {
+        Unknown = -1,
         Rich,
         Link,
         Video,
         Image,
         Gifv,
         Article,
-        Tweet
+        Tweet,
+        Html,
     }
 }