Add more methods in orgevents.py

[sks444]

I was thinking if we could add more direct methods in orgevents.py similar to get_org_open_issues like get_org_open_prs, get_org_closed_issues, get_org_closed_prs.
@staeiou, if you like this feature, i would like to create a PR for this.

[sks444]

Oh, actually we are getting all the open issues by get_org_open_issues which is also covering the open PRs, so we only need one method i.e.: get_org_closed_issues which will also cover merged or closed PRs.