Add PriorityAttribute and sortby priority in Search

src/Discord.Net.Commands/Attributes/PriorityAttribute.cs

@@ -0,0 +1,18 @@
+using System;
+
+namespace Discord.Commands
+{
+    /// <summary> Sets priority of commands </summary>
+    [AttributeUsage(AttributeTargets.Method)]
+    public class PriorityAttribute : Attribute
+    {
+        /// <summary> The priority which has been set for the command </summary>
+        public int Priority { get; }
+
+        /// <summary> Creates a new <see cref="PriorityAttribute"/> with the given priority. </summary>
+        public PriorityAttribute(int priority)
+        {
+            Priority = priority;
+        }
+    }
+}