Plan for version 4.x.x

[pjcdawkins]

Major possible changes:

• Symfony 4
• Require PHP >= 7.1 (at least because of Symfony 4)
• Use of SymfonyStyle to format output
• Use DI autowiring for commands and services (services via type hints rather than getService('id')), and cache the DI container
• PHP 7.1 language features
• New libraries for OAuth 2.0 support (#641)
• Support for newer Platform.sh APIs
• Remove deprecated commands/options/aliases/code

Other possible changes:

• Split integration:add command (to reduce the number of options in help)
• Split or somehow paginate list command
• Better multi support
• Remove --host option
• Use an HTTP (etag-based) cache via the API client, and ditch the manual cache
• Add --no-interaction (alias for --yes)
• ...