[feature]: ADSR (#15)

closes #14

Reviewed-on: #15

inc/Effect.h

@@ -0,0 +1,13 @@
+#pragma once
+#include <vector>
+class Effect {
+  private:
+    /* data */
+  public:
+    Effect(/* args */){};
+    ~Effect(){};
+    virtual void OnSetNote(){};
+    virtual void OnUnsetNote(){};
+    // virtual void RetriggerState(){};
+    virtual void Process(std::vector<float>& samples){};
+};