Added option --pretend to only print the filenames

Added option `--pretend` (or `-e`) to only print the filenames of files to import without actually importing them. 
Rather than use this intermediate `session.pretend` field, we should probably just access `session.config['pretend']` for uniformity. To make this work without an explicit membership check, you can add `pretend: false` to the `import` section of `config_default.yaml`.