Feature request: subcategories

Hello. Wonderful plugin you created! However, I was just confronted with a need that your plugin currently doesn’t address, which is subcategories.

It would be great if I could mark certain rows as being subcategories of the row above it, so that:
1. They can be made visually and hierarchically distinct, for instance with some padding on the left.
2. They can be sorted the normal way, or the category it belongs to get sorted but not its children, or you can sort the children within a category.

Would you consider adding such a feature? Thank you for your consideration.