title: SqlAlchemy Continuum: history and transaction tables are not created
body: <p>I am trying to integrate <em>sqlalchemy-continuum</em> versioning into an existing application that uses flask-sqlalchemy. </p>
  
  <p>I have a <code>__versioned__ = {}</code> to the models I want to have versioned e.g.:</p>
  
  <pre><code>class User(Base):
      __versioned__ = {}
      __tablename__ = 'user'
  </code></pre>
  
  <p>And I am trying to initialize sqlalchemy-continuum as follows:</p>
  
  <pre><code>from flask_sqlalchemy import SQLAlchemy
  from sqlalchemy_continuum import make_versioned
  from sqlalchemy.orm import configure_mappers
  
  class SQLOrm():
      def __init__(self):
          if not hasattr(self, 'orm'):
              make_versioned(user_cls=None)
              self.orm = SQLAlchemy()
              configure_mappers()
  
      # ... 
  
  db_orm = SQLOrm()
  </code></pre>
  
  <p>But the transaction table and history tables are not created. I can see continuum is trying to create records, but there are no tables:</p>
  
  <pre><code>LINE 1: INSERT INTO transaction (issued_at, id, remote_addr) VALUES ...
                       ^
  [SQL: "INSERT INTO transaction (issued_at, id, remote_addr) VALUES (%(issued_at)s, nextval('transaction_id_seq'), %(remote_addr)s) RETURNING transaction.id"] [parameters: {'issued_at': datetime.datetime(2019, 5, 3, 0, 46, 39, 699358), 'remote_addr': None}] (Background on this error at: http://sqlalche.me/e/f405)
  </code></pre>
  
  <p>There is something wrong in the way I am initializing this. Any idea what I am doing wrong?</p>