Added IGuildUser Add/Remove Roles extensions

src/Discord.Net/Extensions/GuildUserExtensions.cs

@@ -0,0 +1,19 @@
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace Discord.Extensions
+{
+    public static class GuildUserExtensions
+    {
+        public static Task AddRolesAsync(this IGuildUser user, params IRole[] roles)
+            => AddRolesAsync(user, (IEnumerable<IRole>)roles);
+        public static Task AddRolesAsync(this IGuildUser user, IEnumerable<IRole> roles)
+            => user.ModifyAsync(x => x.Roles = Optional.Create(user.Roles.Concat(roles)));
+
+        public static Task RemoveRolesAsync(this IGuildUser user, params IRole[] roles)
+            => RemoveRolesAsync(user, (IEnumerable<IRole>)roles);
+        public static Task RemoveRolesAsync(this IGuildUser user, IEnumerable<IRole> roles)
+            => user.ModifyAsync(x => x.Roles = Optional.Create(user.Roles.Except(roles)));
+    }
+}