Allow importing closed issues as well
Allow importing closed issues as well

Using either the `--all` or `--closed` flags.

Plus quite a bit of refactoring.

lukesmith/github-issues-import,abrkn/github-issues-import,danmichaelo/github-issues-import,crawford/github-issues-import,LalatenduMohanty/github-issues-import,nrc/github-issues-import,IQAndreas/github-issues-import,jonboulle/github-issues-import,simonfork/github-issues-import