Configure default plugins

[Nyholm]

There are some plugins in php-http/plugins that got dependencies. We should allow the user to configure those plugins.

httplug: 
  plugins: 
    cache:
      service: 'acme.cache'
      config: 
        respect_headers: true
    logger:
      formatter: 'acme.formatter'

Reference: #16 (comment)

[dbu]

agree. we can use the ->canBeEnabled() construct in configuration to know for each plugin whether it should be loaded. as soon as any other option is configured, enabled becomes true automatically. the DI extension would pass parameters (retry limit, redirect limit, cache service and so on) and remove plugins that are disabled from the container.