Added method to load variant images and 'variant_cost_price' method

[redglory]

1. We only had the possibility to load master variant images so I've added a new method to also be able to load variants images.
2. Added 'variant_cost_price' method as it wasn't present (variants can have different cost/final prices)
3. On existing method 'add_images' changed 'get_each_assoc' which expects a '|' to separate each image into @populator.current_value.to_s.split(Delimiters::multi_value_delim) in order to maintain multiple value logic (separated with a ','). I got confused when loading master variant images because it was expecting a '|'.

[autotelik]

fantastic work. thanks so much. I'm working on the main datashift code to improve the design - all grew from a spree loader script - so in near future should be able to move this kind of thing @we_can_process_these_anyway out of code and into config