Automatic file deduplication

This release introduces subcommands. The old functionality of duplicate file search has been moved to the group subcommand. New subcommands have been introduced: link and remove. Both commands work on a list of files previously generated with group, passed on the standard input. fclones remove removes redundant files, fclones link replaces redundant files with links.

Additionally, a set of options was added to control the selection of files that should be removed. It is possible to select files by their creation time, modification time, last access time, nesting level, or to match them by path / name patterns.