summarize command for GitHub Issues

[patrickdevivo]

Similar to output for mergestat summarize commits and mergestat summarize blame (in format), a mergestat summarize repo-issues owner/repo command:

• Total issues opened
• Total issues closed
• Total authors
• Average time to close
• Average participant count
• Average comment count

Breakdown by Author (GitHub login)

• Login
• Total Issues opened by author in period
• Issues remaining open in period (and avg age?)
• Issues closed in period
• Average time to close

Fetching issues can take time, by default only show a limited time frame (maybe the last 6 months or year)