Added support for animated emoji (#913)

* Added support for animated emoji

This was such a useful feature Discord, I'm glad you added this instead
of fixing bugs.

* Fix bugs in emote parser

* Added unit tests for emotes

test/Discord.Net.Tests/Tests.Emotes.cs

@@ -0,0 +1,44 @@
+using System;
+using Xunit;
+
+namespace Discord
+{
+    public class EmoteTests
+    {
+        [Fact]
+        public void Test_Emote_Parse()
+        {
+            Assert.True(Emote.TryParse("<:typingstatus:394207658351263745>", out Emote emote));
+            Assert.NotNull(emote);
+            Assert.Equal("typingstatus", emote.Name);
+            Assert.Equal(394207658351263745UL, emote.Id);
+            Assert.False(emote.Animated);
+            Assert.Equal(DateTimeOffset.FromUnixTimeMilliseconds(1514056829775), emote.CreatedAt);
+            Assert.EndsWith("png", emote.Url);
+        }
+        [Fact]
+        public void Test_Invalid_Emote_Parse()
+        {
+            Assert.False(Emote.TryParse("invalid", out _));
+            Assert.False(Emote.TryParse("<:typingstatus:not_a_number>", out _));
+            Assert.Throws<ArgumentException>(() => Emote.Parse("invalid"));
+        }
+        [Fact]
+        public void Test_Animated_Emote_Parse()
+        {
+            Assert.True(Emote.TryParse("<a:typingstatus:394207658351263745>", out Emote emote));
+            Assert.NotNull(emote);
+            Assert.Equal("typingstatus", emote.Name);
+            Assert.Equal(394207658351263745UL, emote.Id);
+            Assert.True(emote.Animated);
+            Assert.Equal(DateTimeOffset.FromUnixTimeMilliseconds(1514056829775), emote.CreatedAt);
+            Assert.EndsWith("gif", emote.Url);
+        }
+        public void Test_Invalid_Amimated_Emote_Parse()
+        {
+            Assert.False(Emote.TryParse("<x:typingstatus:394207658351263745>", out _));
+            Assert.False(Emote.TryParse("<a:typingstatus>", out _));
+            Assert.False(Emote.TryParse("<a:typingstatus:not_a_number>", out _));
+        }
+    }
+}