Expand (or document) the import to handle more than goodreads

I recently tried to import some books via a list of ISBNs that I manually compiled from a few different services that held my data in different formats.

There is a functional, but slightly hidden, import option, but currently it expects a goodreads export file and fails if it gets a slightly unexpected format.

If there was documentation of the minimum .csv file needed for this, I could probably have created one to work with what the code expected. In the end I ended up creating an actual Goodreads export just to use the importer.

Goodreads and inventaire.io both accept just ISBNs in text as import formats, and I'd guess the current code could be tweaked with relative ease to allow this as well (I found the import UI of Inventaire.io particularly well thought out, with options for power-users without being overwhelming, and liked that it could be used to quickly build "lists/shelves" too).