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. 
The `importer` module is intended to be decoupled from the command-line interface. (That is, one day, we might add something crazy like a web frontend for the importer!) To that end, please use `log.info` instead of `ui.print_`.