Release 0.20.0

This release introduces a huge performance enhancement: file hash caching.
To enable caching use --cache option. See README.md Tuning section.

What's Changed

• Only use the reflink crate on Linux for tests by @th1000s in #108
• Persistent caching of file hashes by @pkolaczk in #111
• Refactoring: move grouping related stuff to group.rs. by @pkolaczk in #112
• Use fclones::path::Path for GroupConfig::base_dir by @pkolaczk in #114

Full Changelog: v0.19.0...v0.20.0