add a flag to run only schema migrations

[jd]

The use case would be apparent if you are writing code in both production and development environments. In the dev environment, you would only want to run schema migrations, whereas in production you would want to run both schema and data migrations.

[andrew]

I'm not sure the use case you present is particularly compelling; why would you not want data migrations in development? You need to run them against test data anyway to ensure they work, and in addition, it saves time if you don't have to redo the database from scratch each time.