Accept directories extracted from original compressed files

Currently irds requires the original files from a dataset, such as a tar.gz file for the NYT corpus. It would be nice if the extracted directory could also be provided as input instead. This would be useful in cases where the original compressed file wasn't kept (as happened to me with NYT). Taking the directory as input is also closer to what most other tools do, so it potentially removes the need to have both the compressed file and its contents available.