Provide a way to set application environment

[BoShurik]

Now there is a note to not rebuild config in production but there is no method to set environment

[samdark]

How about using environment variable as in https://github.com/yiisoft/yii-console/blob/master/bin/yii#L48?

[BoShurik]

@samdark Yes, but this is not enough. Often I need different services definitions for different environment (e.g. logger). It would be great to provide standard way to do that. As I understand composer config plugin support this

[rustamwin]

See https://github.com/yiisoft/config/pull/64