Add command for computing breakdown of issues by age

[izuzak]

It would be nice to add a new command (e.g. called breakdown) which gives a breakdown of issues by age. For example, the command would determine the number of open issues older than 2 years, 1 year, 6 months, 1 month, and 1 week. The command should support the same scopes and filters as the program does now, so that it's possible to compute these statistics for a specific repository, organization, or a specific label.