version 0.2.0

Changes in version 0.2.0 (most should be backwards compatible, but some edge cases may have changed):

• added progress bars for downloads
• added (optional) inheritance of parameters from parent corpora to their children
• added general keyword mappings to map string values from config to arbitrary objects (e.g. loader functions)
• added support for archived and compressed corpora in zip/tar files
• unified API for FileCorpus, ZipFileCorpus, and TarFileCorpus; potential breaking change: files() iteration returns path-like objects that do NOT conform to os.PathLike as paths within archive files do not exist in the file system; use path.open(...) instead of open(path, ...) to avoid problems
• deprecated config.set in favour of config.set_values
• some other fixes/improvements