Make timestamp format of `@readable_time` configurable by YAML

IMO `YYYY-MM-DD-HH-MM-SS` is more readable than `YYYYMMDDHHMMSS`, but it's depends on sense of individual.
So it seems better idea to make configurable timestamp format by repository-specific YAML file.

How do you think?

Screen capture: comparison of two types of timestamp format in pull request view http://gyazo.com/b9c67c26b90d5949f1580f25ffbcfd2e