Toggling between `alembic` databases

Alembic for different envs

`alembic` is great but lacks an out of the box way to set up running migrations against a specific database (e.g. `development`, `test`, `production`). The following adjustments to its `env.py` and `alembic.ini` allow us to target a specific database:

**Example:**

```bash
alembic -x db=development upgrade head
```

**env.py:**

```python
target_metadata = None

cmd_kwargs = context.get_x_argument(as_dictionary=True)
if 'db' not in cmd_kwargs:
    raise Exception('We couldn\'t find `db` in the CLI arguments. '
                    'Please verify `alembic` was run with `-x db=<db_name>` '
                    '(e.g. `alembic -x db=development upgrade head`)')
db_name = cmd_kwargs['db']
# ... 
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.

    """
    # Overload db config on top of alembic config
    alembic_config = config.get_section(config.config_ini_section)
    db_config = config.get_section(db_name)
    for key in db_config:
        alembic_config[key] = db_config[key]

    engine = engine_from_config(
        alembic_config,
        prefix='sqlalchemy.',
        poolclass=pool.NullPool)
```

**alembic.ini:**

```
[alembic]
# ...

[development]
sqlalchemy.url = postgresql://localhost/dev

[test]
sqlalchemy.url = postgresql://localhost/test

[production]
sqlalchemy.url = postgresql://production/prod
```