Adds default-environment option

[nunomaduro]

This pull request adds the default-environment option to the vapor.yml file. In other words, people can now set their default environment using the code bellow, and any command like vapor deploy, vapor up, vapor secret:list, will use the selected default environment instead of staging.

id: 1
name: app
default-environment: production
environments:
    production:
        build:
            - 'COMPOSER_MIRROR_PATH_REPOS=1 composer install --no-dev'
    qa:
        build:
            - 'COMPOSER_MIRROR_PATH_REPOS=1 composer install'

This is useful, as there is a lot of people who don't have staging environment.