add: ability to hook into cli urilities

the patch introduces way for package developers to hook into CLI
utilities (e.g. cli utility sending email notifications) by creating a
newly introduced `cli` plugin type which can provide `onCli` event
handler

see http://www.getanahita.com/topics/123375-bug-unable-to-hook-into-code-executed-from-command-line

The patch cannot result in any conflicts because it will load new type of plugins, the only issue may be is that _JPluginHelper::importPlugin()_ can raise strict notifications when error_reporting includes E_STRICT, but this is system wide joomla legacy issue, doesn't relate to this patch in particular.