Common Alembic environment runner

Make sqlalchemy.alembic_env with Env. Env is a base environment runner class. Env provides basic functionality to configure environment and run revision scripts. Project can use Env class as is, if it much project’s purposes. Otherwise project can inherit the class end override a methods. Env contains same functionality as default env.py module, but it is decomposed into several methods, intended to configure different objects needed by an environment.