Add genre as a variable folder structure when downloading.

[rkingett]

Very similar to #838 but I would also like the ability to include genres in the list of the naming templates

At present, there is no naming template that includes category and or genre. This would be a really great edition for folks that want to organize their downloads a bit more granular.

Ideally, I'd like the variable to be called something like,

<genre>

Additional context

I am unsure how deep the program can look for the Audible ID to be sure we downloaded the book but if it can't search beyond a certain level deep, that would be good to note in the documentation.