Add example archiver and CoreConsole task to generate Archiver into plugin.

Refs #6516 
Maybe you can add some links here where to get more information about archiver similar to https://github.com/piwik/piwik/blob/master/plugins/ExampleCommand/Commands/HelloWorld.php or https://github.com/piwik/piwik/blob/master/plugins/ExamplePlugin/Widgets.php . I am not sure if we have any docs or API reference on developer.piwik.org so if not then just ignore.