Provide administrative method to enable/disable skins

Currently the capacity is there to enable/disable a skin, but only programmatically using default status:

$skins['fusion_skins_block_positioning'] = array(
  'title' => t('Block position'),
  'description' => t('Change the position of this block (default is to float to the left)'),
  'default status' => TRUE,
  // ...
);

Sites with a lot of skins will need an administrative method to toggle them on/off, otherwise the "edit skin" screen will become overwhelmed.