Add mentionable property to ModifyGuildRoleParams

Allows roles to be configured to be mentionable, fixes #424

src/Discord.Net.Core/API/Rest/ModifyGuildRoleParams.cs

@@ -16,5 +16,7 @@ namespace Discord.API.Rest
         public Optional<uint> Color { get; set; }
         [JsonProperty("hoist")]
         public Optional<bool> Hoist { get; set; }
+        [JsonProperty("mentionable")]
+        public Optional<bool> Mentionable { get; set; }
     }
 }

src/Discord.Net.Core/Entities/Roles/ModifyGuildRoleParams.cs

@@ -47,5 +47,12 @@
         /// If this role is the EveryoneRole, this value may not be set.
         /// </remarks>
         public Optional<bool> Hoist { get; set; }
+        /// <summary>
+        /// Whether or not this role can be mentioned.
+        /// </summary>
+        /// <remarks>
+        /// If this role is the EveryoneRole, this value may not be set.
+        /// </remarks>
+        public Optional<bool> Mentionable { get; set; }
     }
 }