(ifcbrk)fix: #1335 Add isMentionable parameter to CreateRoleAsync in non-breaking manner (#1418)

* Fix #1335 Add isMentionable parameter to CreateRoleAsync in non-breaking manner

This PR adds the isMentionable parameter to the CreateRoleAsync method
in a way that prevents it from being interface-breaking. This has been done
by adding it as an optional parameter at the end of publicly-exposed
methods.

This parameter determines if the newly created role can be mentioned as it is
created.

* Overload CreateRoleAsync methods

src/Discord.Net.Rest/Entities/Guilds/GuildHelper.cs

@@ -257,7 +257,7 @@ namespace Discord.Rest
         //Roles
         /// <exception cref="ArgumentNullException"><paramref name="name"/> is <c>null</c>.</exception>
         public static async Task<RestRole> CreateRoleAsync(IGuild guild, BaseDiscordClient client,
-            string name, GuildPermissions? permissions, Color? color, bool isHoisted, RequestOptions options)
+            string name, GuildPermissions? permissions, Color? color, bool isHoisted, bool isMentionable, RequestOptions options)
         {
             if (name == null) throw new ArgumentNullException(paramName: nameof(name));
 
@@ -270,6 +270,7 @@ namespace Discord.Rest
                 x.Permissions = (permissions ?? role.Permissions);
                 x.Color = (color ?? Color.Default);
                 x.Hoist = isHoisted;
+                x.Mentionable = isMentionable;
             }, options).ConfigureAwait(false);
 
             return role;