Populate EnvironmentContext with alembic.ini config

because I want to be able to set `version_table_schema` in `alembic.ini`
and have it propagate.

Previously, I had to have the following in my `env.py`:

```python
def run_migrations_online():
    """Run migrations in 'online' mode.

    In this scenario we need to create an Engine
    and associate a connection with the context.

    """
    engine = engine_from_config(
                config.get_section(config.config_ini_section),
                prefix='sqlalchemy.',
                poolclass=pool.NullPool)

    connection = engine.connect()
    alembic_ini_config_dict = config.get_section(config.config_ini_section)  # <-- Stuff I added
    context.configure(
                connection=connection,
                target_metadata=target_metadata,
                **alembic_ini_config_dict  # <-- Stuff I added
                )

    try:
        with context.begin_transaction():
            context.run_migrations()
    finally:
        connection.close()
```

With the changes in this PR, I can use the value in `alembic.ini` without having to hack my `env.py` file.