automatically closing stale pull requests

[notna via Digitalmars-d]

another interesting discussion [1]... and old/stale pull requests 
are also discussed here now and then... not sure if this [2] is 
known to many ppl?!

- [1] https://marc.info/?t=153926658700001&r=1&w=2
- [2] https://github.com/probot/stale