Add an option to include closed issues for GitHub

[pawamoy]

With the new githubclosedon UDA added in #621, I would now like to add an option to the GitHub service to also pull closed issues, not only open ones.

@wookietreiber pointed me to these two lines

https://github.com/ralphbean/bugwarrior/blob/2841f05ae66a0d8f8c91c03ec896cad6b8d2cb0a/bugwarrior/services/github.py#L48

https://github.com/ralphbean/bugwarrior/blob/2841f05ae66a0d8f8c91c03ec896cad6b8d2cb0a/bugwarrior/services/github.py#L59

...where we should add a state=all parameter depending on a github.include_closed_issues configuration option. Maybe they are not the only ones we need to change, I will see that.

Will open a PR soon.

[pawamoy]

Well, the PR was closed (by me), so, closing this as well. Feel free to reopen 🙂