Add Custom tags to sort capabilities.

I think it would be helpful to have the ability to add a custom sort option. For instance, my brother and I have a joint collection and I'd like to be able to see which items are his, which are mine, and which are mutual. Or if I wanted to label all the games I wanted to sell, or games that I need to focus on finding better conditioned copies for. I saw a few items in other suggestions that I think a custom labeling system could solve.