title: Alembic/Flask-Migrate not detecting after_create events
body: <p>I have a simple Flask-SQLAlchemy model (with event listener to create trigger):</p>
  
  <pre><code>from flask_sqlalchemy import SQLAlchemy
  
  db = SQLAlchemy()
  
  class Confirm(db.Model):
    created = db.Column(db.DateTime, default=db.func.current_timestamp(), nullable=False)
    modified = db.Column(db.DateTime, default=db.func.current_timestamp(), onupdate=db.func.current_timestamp(), nullable=False)
    id = db.Column(db.String(36), primary_key=True) 
  
  class ConfirmOld(db.Model):
    orig_created = db.Column(db.DateTime)
    orig_modified = db.Column(db.DateTime)
    orig_id = db.Column(db.String(36))
  
  confirm_delete = DDL('''\
  CREATE TRIGGER confirm_delete 
  BEFORE DELETE
    ON confirm FOR EACH ROW
    BEGIN
      INSERT INTO confirm_old ( orig_created, orig_modified, orig_id )
      VALUES ( OLD.created, OLD.modified, OLD.id );
    END;
  ''')
  
  event.listen(Confirm.__table__, 'after_create', confirm_delete)
  </code></pre>
  
  <p>When I run Alembic migrate and upgrade, the TRIGGER is not created (in MySQL). However, it <em>is</em> created and works properly when I use db.create_all().</p>
  
  <p>Is it possible to get Alembic / Flask-Migrate to create and manage my triggers (i.e., custom DDL that is run on after_create events)?</p>