add config for probot-stale integration

[boneskull]

closes #2828.

Issues and PRs will be marked stale after 4 mos. of inactivity and closed if there's no activity after 2 more weeks.

Most issues with labels are exempt from the check. I'm not sure how well this is going to work, but we can always change it later.

[coveralls]

Coverage remained the same at 88.288% when pulling 468f791 on stale into fc35691 on master.

[dasilvacontin]

I'm finding it hard to understand which issues are worthy of being marked as stale and being closed.

I can come up with:

• help issues that ask for help with a concrete code example, we reply to them, and we never hear back from OP.

Any others?

[boneskull]

@dasilvacontin IMO, we should mark the following as stale:

• PRs that need work and have become rotten
• Stuff marked needs-feedback that never got feedback
• Unlabeled stuff without activity

4 mos is a long time, and there's a warning, so a user can reply and say "yes, this is still a question/problem/etc".

In the config, we're blacklisting almost every label except a few (duplicate, needs-feedback, pr-needs-work). Really, we need a whitelist instead (see probot/stale#22).