Change to check issue id and update_at

Hello, Thank you for great extension.
Our team has begun to use this extension.
But, I'm facing two issues.

1. I'd like to check some repositories. but, the extension check to issue number.
I think that's better if it check in the issue id.
2. If other member updates the issue, I want to receive notification every time. 
So, this pull request checked the issues's update time.

Sincerely.
Eiji