Add import from Bookshelf

**Describe the solution you'd like**
There is a quite popular mobile App called [Bookshelf](https://play.google.com/store/apps/details?id=com.bookshelf.prod). It would  be nice to have an import function for *booktrack-cli*.

Add another subcommand *import* maybe with some parameters for diferrent formats. It has to be decided what to do with duplicates.
