Add plugins

In the past we had the plugins in a separate repository. From now on, the core plugin implementations and the plugin client itself are going to be found in this package, while plugins with external dependencies are extracted to separate packages:

https://github.com/php-http/logger-plugin
https://github.com/php-http/cache-plugin
https://github.com/php-http/stopwatch-plugin

First stable release

Remove unnecessary prefer-stable

Only required when there are development stability deps.

Initial release

v0.1.0

Prepare release, fixes php-http/httplug#99