Conditionally require tax category for products 

Hi @Matt-Yorkley,

I've had a bit of a play with setting up a config variable to store whether we want to require products to have a tax_category or not.

Have a look around and let me know if you have any questions. If you're happy, pull in my changes here, and that should automatically update your pull request to for the main repository, where I can pull in all of our combined changes. Bit of a weird workflow I know, but I think it makes sense once you get your head around it.

For future reference, its a good idea to create a dedicated branch in this repository for each pull request you want to make across to the main one, that way you avoid accidentally allowing changes made in your master branch here to flow through to your pull request (which happens automatically...it's black magic....).

Thanks!

Rob