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/Entities/Messages/EmbedBuilder.cs

@@ -137,6 +137,17 @@ namespace Discord
             Author = author;
             return this;
         }
+        public EmbedBuilder WithAuthor(string name, string iconUrl = null, string url = null)
+        {
+            var author = new EmbedAuthorBuilder
+            {
+                Name = name,
+                IconUrl = iconUrl,
+                Url = url
+            };
+            Author = author;
+            return this;
+        }
         public EmbedBuilder WithFooter(EmbedFooterBuilder footer)
         {
             Footer = footer;
@@ -149,6 +160,16 @@ namespace Discord
             Footer = footer;
             return this;
         }
+        public EmbedBuilder WithFooter(string text, string iconUrl = null)
+        {
+            var footer = new EmbedFooterBuilder
+            {
+                Text = text,
+                IconUrl = iconUrl
+            };
+            Footer = footer;
+            return this;
+        }
 
         public EmbedBuilder AddField(string name, object value)
         {
@@ -185,6 +206,17 @@ namespace Discord
             this.AddField(field);
             return this;
         }
+        public EmbedBuilder AddField(string title, string text, bool inline = false)
+        {
+            var field = new EmbedFieldBuilder
+            {
+                Name = title,
+                Value = text,
+                IsInline = inline
+            };
+            _fields.Add(field);
+            return this;
+        }
 
         public Embed Build()
         {

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);
+    }
+}