text: Title: add a timestamp token that can be part of the file_template
  Body: Wouldn't it be nice to have the migration files (the ones which are in the `migration/versions` folder) sorted by creation date. These are hard to retrieve without the help of the command `alembic history`.
  
  I propose a new variable to use in the `file_template` option, that could be placed at first, so that the file order could be maintained.
  
  Is that reasonable and isn't that somehow a shallow request ?
  