compression enhancements:

 * added `compression` kwarg to compress_and_encyrpt{,_path} and
   decrypt_and_uncompress

 * introduced snappy (de)compression

 * introduced gzip compression

 * harmonized API with existing xz compression

 * added tests

 * helper function `parse_file_extensions`
Merge branch 'master' of github.com:trec-kba/streamcorpus