feat: add author filtering

[mloncarevich]

Summary

• Add --author / -a flag to filter commits by author name
• Case-insensitive partial match (e.g., --author "milan" matches "Milan Loncarevic")
• Can be combined with date filters
• Refactored filter logic for cleaner code

Usage

# Filter by author
gitstats stats --author "Milan"

# Combine with date range
gitstats stats --author "Milan" --since 2025-01-01

# With JSON output
gitstats stats --author "Milan" --json

Example output

📊 Git Statistics for: .
Filtered: author: Milan

Total commits: 42
Contributors: 1
...

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