Allow specifying `state` in `get_issues()`
Allow importing closed issues as well

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

Plus quite a bit of refactoring.
Add a notice in the title if an issue has been closed

A bit of a workaround, since detecting if an issue has been
closed/merged/reopened is a bit of a workaround.

Also, as far as I can see, there is no way to close an issue from the
GitHub API. But I'll ask around and see if it is possible.
Update README to reflect the new changes
Add ability to import closed issues as well

Uses the following flags: `--all`, `--open`, `--closed`, `--issues 1 2 3 4 [...]` (one of which is always required).