add onmeeting event

MiraAPI/Modifiers/BaseModifier.cs

@@ -104,6 +104,13 @@ public abstract class BaseModifier : IOptionable
     {
     }
 
+    /// <summary>
+    /// Called when a meeting starts.
+    /// </summary>
+    public virtual void OnMeetingStart()
+    {
+    }
+
     /// <summary>
     /// Determines whether the player can vent.
     /// </summary>