Add `number_with_delimiter` to long counters

Makes long counters easy to read:

![commits](https://cloud.githubusercontent.com/assets/10076/5302395/cc1008c0-7b94-11e4-9bf3-9959a763895b.png)
![commit](https://cloud.githubusercontent.com/assets/10076/5302396/cc134382-7b94-11e4-844c-ae24ca630f37.png)
![releases](https://cloud.githubusercontent.com/assets/10076/5302398/cc15434e-7b94-11e4-97c2-c1d9ac4cb459.png)
![contributors](https://cloud.githubusercontent.com/assets/10076/5302397/cc14d27e-7b94-11e4-9ad6-e515996f896c.png)

@fxn What do you think?