Tax Categories for Product Variants

Currently you can only set a tax category on a product as a whole, this is largely fine but here in the UK there are a number of occasions when different variants of a product will require different VAT (tax) treatment. An example of this is as follows:

> Children's clothing in UK VAT is zero rated (0% tax) but adults clothing is standard rate (20% tax). A clothing store that sells the same t-shirt design in sizes ranging from children's to adults does not want to make two separate products for the same t-shirt as this would be duplication.

Whilst this might seem a rather specific issue I wanted to suggest this as a non-breaking change as it will fallback to the product tax category if there isn't one on the variant itself so if you do not need to set the tax at the variant level you don't have to.

**Disclaimer:** This is a work in progress at the moment as I need to add specs for the changes but would like to get others more experienced with Spree's inner workings input in the meantime.