Add configuration for probot-stale bot

[rnorth]

Adding stale bot. It's already added to the project. Once this is merged, it will become active.

I've tweaked the example configuration to use some custom timings and comments:

• 90 days stale, with 14 days between the stale warning and the issue being closed
• I've tried to keep the comments actionable so that we get positive results out of doing this.

I think that over time we can reduce the time limits, but from our current starting position we should be quite generous.

WDYT?

[rnorth]

Enabling now.