Add param "option" for RTable::getAutoInstance

Add this param "option" for allow 1 extension can getInstance a table from another extension. This option already have in RModel, I think RTable and RTableNested nice to have this too.

Ex: In redITEM view, can get an instance of RedsliderTableGallery by this line:
$redsliderGalleryTable = RTable::getAdminInstance('Gallery', array('ignore_request' => true), 'com_redslider');