Add embed builder extensions

People in #dotnet_discord-net suggested that this should be part of
the lib after I demonstrated it

src/Discord.Net.Rest/Extensions/EmbedBuilderExtensions.cs

@@ -0,0 +1,43 @@
+namespace Discord
+{
+    public static class EmbedBuilderExtensions
+    {
+        public static EmbedBuilder AddField(this EmbedBuilder builder, string title, string text, bool inline = false) =>
+            builder.AddField(field =>
+            {
+                field.Name = title;
+                field.Value = text;
+                field.IsInline = inline;
+            });
+
+        public static EmbedBuilder WithFooter(this EmbedBuilder builder, string text, string iconUrl = null) =>
+            builder.WithFooter(footer =>
+            {
+                footer.Text = text;
+                footer.IconUrl = iconUrl;
+            });
+
+        public static EmbedBuilder WithColor(this EmbedBuilder builder, uint rawValue) =>
+            builder.WithColor(new Color(rawValue));
+
+        public static EmbedBuilder WithColor(this EmbedBuilder builder, byte r, byte g, byte b) =>
+            builder.WithColor(new Color(r, g, b));
+
+        public static EmbedBuilder WithColor(this EmbedBuilder builder, float r, float g, float b) =>
+            builder.WithColor(new Color(r, g, b));
+
+        public static EmbedBuilder WithAuthor(this EmbedBuilder builder, string name, string iconUrl = null, string url = null) =>
+            builder.WithAuthor(author =>
+            {
+                author.Name = name;
+                author.IconUrl = iconUrl;
+                author.Url = url;
+            });
+
+        public static EmbedBuilder WithAuthor(this EmbedBuilder builder, IUser user) =>
+            WithAuthor(builder, $"{user.Username}#{user.Discriminator}", user.AvatarUrl);
+
+        public static EmbedBuilder WithAuthor(this EmbedBuilder builder, IGuildUser user) =>
+            WithAuthor(builder, $"{user.Nickname ?? user.Username}#{user.Discriminator}", user.AvatarUrl);
+    }
+}