Suggest to add Stale bot in this repo

[xiaohk]

Hey @interpret-ml:

Thank you so much for promptly replying all the issues here! The detailed responses and discussions are very insightful. Reading through some recent issues, I found that many of them had been resolved but remained open. I suggest to deploy the Stale bot in this repo. It would automatically label issues as stale and close them if no further activity after some certain period of time. It will make it easier to track new and unresolved issues.