[FEATURE] Add option to display PRs in prs stats

[dblock]

Is your feature request related to a problem?

Sometimes we want to see actual PRs in ./bin/project prs stats.

What solution would you like?

Occasionally I copy-paste this to display PRs by external contributors. I want to be able to specify the bucket(s) and the max number to display.

if prs[:external]&.size&.< 100
  puts ''
  prs[:external]&.each do |pr|
    puts pr
    end
  end
end