relative time parsers and period specification

What's New

• Relative date parsing for --from/--to flags: now, now-30d, now-4w, now-2m, now-1y
• Predefined --period flag: yesterday, this_week, last_week, this_month, last_month, this_year, last_year
• GetDatesFromPeriod() library helper for Go package consumers
• All CLI commands now output consistent JSON (replaced gout with encoding/json)
• Fixed errcheck lint errors on output.Write calls
• Fixed test isolation issues with cobra flag state
• Updated documentation for Go library and CLI usage