Support the -x flag in env.py

[luolingchun]

Support changing the default schema of postgres during migration.

By changing search_path to implement, refer to Rudimental Schema-Level Multi Tenancy for PostgreSQL Databases.

Usage：

flask db migrate -x schema=my_schema

[miguelgrinberg]

There are many ways to manage multiple tenants and/or schemas. Given that Alembic does not offer any solution, I prefer to align with it and do the same. The env.py file is supposed to be customized to meet your needs, so these changes are something you should do in your own project, there is no need for them to be in this package.