Add an env-get command

[EtienneM]

scalingo env-get RAILS_ENV

$ scalingo help env-get
NAME:
   scalingo env-get - Get the requested environment variable from your app

USAGE:
   scalingo env-get [command options] variable-name

CATEGORY:
   Environment

DESCRIPTION:
   Get variables:

    $ scalingo -a myapp env-get VAR1

    # See also commands 'env'

OPTIONS:
   --app value, -a value  Name of the current app (default: "<name>")
   --region value         Name of the region to use

$ scalingo -a myapp env-get APP RAILS_ENV
production