Add embed builder extensions (#460)

* Add embed builder extensions

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

* Move some extensions into EmbedBuilder [2]

Apparently git didn't like that previous commit

* Fix error with EmbedBuilderExtensions

A summary of issues which happened:
- Git decided to add an amend commit (I told it to quit?)
- VS Code thinks everything is an error so it wasn't helpful
- dotnet decided to think there was no error until I deleted all
  build outputs and rebuild

Sometimes I question my ability to use version control properly.

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

@@ -0,0 +1,20 @@
+namespace Discord
+{
+    public static class EmbedBuilderExtensions
+    {
+        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, IUser user) =>
+            builder.WithAuthor($"{user.Username}#{user.Discriminator}", user.AvatarUrl);
+
+        public static EmbedBuilder WithAuthor(this EmbedBuilder builder, IGuildUser user) =>
+            builder.WithAuthor($"{user.Nickname ?? user.Username}#{user.Discriminator}", user.AvatarUrl);
+    }
+}