Fix issues with extracting data from multiple files (part files)

- Fixed some errors when references to config.source in the code
was not handled correctly in case of regular expressions (errors
reported by @jpluorange in #90)
- Add comments to the default download and extraction config files
to clarify when and how to use regular expressions
Merge pull request #117 from ninniuz/fix_multiple_files_download_extraction

Fix issues with extracting data from multiple files (part files)