ModifyAsync will accept an EmbedBuilder

**This is not a breaking change**.

This change begins to implement #379, where IUserMessage.ModifyAsync will now accept a `Discord.ModifyMessageParams` func over a `Discord.API.Rest.ModifyMessageParams` func.

src/Discord.Net.Core/Entities/Messages/IUserMessage.cs

@@ -1,5 +1,4 @@
-using Discord.API.Rest;
-using System;
+using System;
 using System.Threading.Tasks;
 
 namespace Discord

src/Discord.Net.Core/Entities/Messages/ModifyMessageParams.cs

@@ -0,0 +1,12 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace Discord
+{
+    public class ModifyMessageParams
+    {
+        public Optional<string> Content { get; set; }
+        public Optional<EmbedBuilder> Embed { get; set; }
+    }
+}