Code without SQLAlchemy dependency

[d0rich]

As I see, this plugin is targeted primarily on the PostgreSQL users. For this DBMS we can't avoid external dependencies, so the generated code uses SQLAlchemy.

However, SQLite support is a part of Python standard library, and for this DBMS we can generate code with no dependencies.

It would be nice to have no-dependencies mode by default, if SQLite is used. Or at least have an option to exclude SQLAlchemy from the generated code.