Add IEmbed#ToEmbedBuilder extension method

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

@@ -1,3 +1,5 @@
+using System;
+
 namespace Discord
 {
     public static class EmbedBuilderExtensions
@@ -19,5 +21,31 @@ namespace Discord
 
         public static EmbedBuilder WithAuthor(this EmbedBuilder builder, IGuildUser user) =>
             builder.WithAuthor($"{user.Nickname ?? user.Username}#{user.Discriminator}", user.GetAvatarUrl());
+
+        public static EmbedBuilder ToEmbedBuilder(this IEmbed embed)
+        {
+            if (embed.Type != EmbedType.Rich)
+                throw new InvalidOperationException($"Only {nameof(EmbedType.Rich)} embeds may be built.");
+
+            var eb = new EmbedBuilder
+            {
+                Title = embed.Title,
+                Description = embed.Description,
+                Url = embed.Url,
+                ImageUrl = embed.Image?.Url,
+                ThumbnailUrl = embed.Thumbnail?.Url,
+                Timestamp = embed.Timestamp
+            }
+            .WithAuthor(embed.Author?.Name, embed.Author?.IconUrl, embed.Author?.Url)
+            .WithFooter(embed.Footer?.Text, embed.Footer?.IconUrl);
+
+            if (embed.Color.HasValue)
+                eb.WithColor(embed.Color.Value);
+
+            foreach (var field in embed.Fields)
+                eb.AddField(field.Name, field.Value, field.Inline);
+
+            return eb;
+        }
     }
 }