add --summary option to only show dedup stats

[kapitainsky]

# fclones group --summary .
[2022-09-07 08:18:03.345] fclones:  info: Started grouping
[2022-09-07 08:18:03.397] fclones:  info: Scanned 14968 file entries
[2022-09-07 08:18:03.398] fclones:  info: Found 12215 (36.7 GB) files matching selection criteria
[2022-09-07 08:18:03.408] fclones:  info: Found 4929 (334.7 MB) candidates after grouping by size
[2022-09-07 08:18:03.410] fclones:  info: Found 4929 (334.7 MB) candidates after grouping by paths
[2022-09-07 08:18:03.451] fclones:  info: Found 2183 (184.4 MB) candidates after grouping by prefix
[2022-09-07 08:18:03.453] fclones:  info: Found 2183 (184.4 MB) candidates after grouping by suffix
[2022-09-07 08:18:03.739] fclones:  info: Found 2159 (107.5 MB) redundant files
# Report by fclones 0.27.2
# Timestamp: 2022-09-07 08:18:03.741 +0100
# Command: fclones group .
# Base dir: /Users/kptsky/Temp/TEST
# Total: 206804715 B (206.8 MB) in 2637 files in 478 groups
# Redundant: 107465883 B (107.5 MB) in 2159 files
# Missing: 0 B (0 B) in 0 files

when I am only interested to see how many duplicates I have without listing all files' details