Add command for computing breakdown of issues by different issue attributes

[izuzak]

This is a generalization of #2. Some examples of breakdowns:

• by age
• by created_at
• by resolution time (how long it took to close the issue or pull request)
• by label
• by creator
• by time to first comment or label from maintainer

For each set of issues defined with a scope and label, the program would determine a set of buckets for which data would need to be collected (e.g. buckets by age, popular labels or creators) or allow the user to input those, and then the stats would be collected.

[simonebracaloni]

Hello, this is a really nice project. Would it be possibile to breakdowns by board status?
We use Github project a lot, and we would love to know how many issue are in a certain column at a given time.

[izuzak]

Hey, @simonebracaloni 👋 Thanks for reaching out! Yeah, that should be possible to implement, but I won't have time to work on such a feature anytime soon. If you'd like -- feel free to try implementing it yourself and opening a PR (I'd be happy to review!). Also, if you have a need for some custom metrics -- you might also consider implementing that yourself via a script which uses the GitHub API.