[RFE] Add EPICs notifications based on last update date

[RomanBednar]

Our team is considering options for optimizing time spent with our epic rundown meetings. The triagebot has been very helpful so far in helping us with bug rundown meetings and efficiently reduced the time spent on the meetings.

The main feature we're looking for is to make the bot notify epic owner if it was not updated in past X days. The bot would need to have new configuration for the epic components, time limit for the notification, and perhaps another field for setting a separate Slack channel.

With this I'd like to propose an enhancement for the triagebot and maybe see if other teams might be interested as well.

[bgilbert]

This isn't functionality we'd need on the CoreOS team, so it's not something I'll be working on myself. Arguably the functionality should go in a separate bot, but setting up a bot isn't trivial, so I'd consider merging a PR if you'd like to add it as a feature here.

I think it'd be more consistent with the triagebot design for it to post a report to a channel listing the active epics and the time since they were last updated. The bot generally tries to provide actionable information without taking a position on whether something 'should have happened by now'.