Feature request/idea: add support for a folder description field

First, thanks for releasing this project. It is extremely well-timed for a project we are working on at the moment and will save us a ton of work :gift_heart:.

For the purposes of our project, we would like to also provide a place for users to describe their folder. I am curious if this feature might be desirable for users outside of our project. 

Side note: apologies, I am new to rails engines and haven't had a chance to grok engine testing just yet. I will add some tests to my fork this coming week. But, I figured I'd run this feature by you before I got too far down the road in order to just to see if there was any interest in adding this to BL folders.

Alternatively, strategies other than including  this or other feature extensions directly into BL folders are also welcomed and appreciated (e.g. a BL Folders "extras" engine that I include along side BL folders vs directly customizing them in our app).

Thanks again, and happy holidays.