refactor codes
Update README.md
add `max_key`, `min_key`, `avg_key` options
Merge pull request #4 from sonots/refactor

Refactoring
Merge pull request #5 from sonots/max

add `max_key`, `min_key`, `avg_key` options
group_by_placeholders
fix for 1.9.3
Update out_groupcounter.rb
rebased