Add concurrency to blamethrower

 1. enqueue all the filenames
 2. fork up to 10 processes that work on individual files
 3. combine the work into a result set

 Improves performance by ~3x on Everlane's codebase
 150 seconds => 50 seconds
formatting like original
Merge pull request #3 from thenanyu/concurrent-process

Concurrent process Thanks @thenanyu!