Added
- `--start-date` and `--end-date` option to `download` command
- `--start-date` and `--end-date` option to `library export` and `library list` command
- better error handling for license requests
- verify that a download link is valid
- make sure an item is published before downloading the aax, aaxc or pdf file
- `--ignore-errors` flag of the download command now continue, if an item failed to download