Add new feature.

Added ILogger;
Added IGraphProcessor;

Changed return type of OnExecute method in BaseNode from void to bool;

Runtime/Contracts/ILogger.cs

@@ -0,0 +1,14 @@
+using System;
+using UnityEngine;
+
+namespace Misaki.GraphView
+{
+    public interface ILogger
+    {
+        public Action<BaseNode, string, LogType> OnLog { get; set; }
+        
+        public void LogInfo(BaseNode node, string message);
+        public void LogWarning(BaseNode node, string message);
+        public void LogError(BaseNode node, string message);
+    }
+}