Extend env-init command to include all customizable values in .env file

[davidalger]

Opened by request from @talesh:

5. Add a flags to warden env-init to define things like PHP version so it'll automatically add PHP_VERSION=7.1 to the .env file
   ref: https://twitter.com/_talesh/status/1182758060546088962

Additional Notes:

• This seems like a very good idea, and can be easily implemented in a BC fashion as env-init is not generally run beyond the initial execution to setup a project.
• May require some work on argument parsing logic to support this.
• This would allow for changing the default versions of things in subsequent releases of Warden without breaking a project's reliance on a particular version since they would all be frozen in .env
• Ideally would include all relevant customizable variables as noted here: https://github.com/davidalger/warden#customizing-an-environment