Bug fixes and new options

• Handles more edge cases and fixes various bugs. In particular, it is now possible to rename something like files 0.jpg, 1.jpg to 1.jpg, 2.jpg (previously, that wouldn't work) or to swap the names of two files.
• Added option "include_directories" to allow including/excluding directories from text buffer.
• Added option "include_header" to allow including/excluding instruction header from text buffer.
• Added command massren --help --config to list possible config values and defaults.