Add .env file support

[dynasource]

Moved from yiisoft/yii2#13602

Original text:
I suggest to add .env support, like it's done in Laravel. With it it's much easier to make a basic
config of the app/switching between environments, etc.

[cebe]

duplicate of #105

[luislobo]

@cebe #105 is a different issue. That one is about handing environment in basic just like it is done in advanced. This other one is for supporting the use of .env files for setting environment variables, as discussed in yiisoft/yii2#13602

[schmunk42]

For reference ... We're using this extension for some years now: https://github.com/vlucas/phpdotenv

Usage examples:

• https://github.com/dmstr/docker-yii2-app/blob/master/src/config/env.php
• https://github.com/dmstr/phd5-app/blob/master/src/config/env.php

[samdark]

Won't be added to 2.0.14. Planned for 2.1.