summarize command for GitHub PRs

[patrickdevivo]

See #256

Total PRs Opened
Total PRs Merged
Total PRs Closed
Total Authors
Avg. Comment Count
Avg. Commit Count
Avg. Files Modified
Avg. Lines added/removed (maybe all these on one line?)
Avg. Time to merge

Breakdown by Author (GitHub login)
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Total PRs opened by author in period
Total PRs merged in period (belonging to author)
Total PRs remaining open in period (and avg age?)
Avg time to merge