Issue #420 - Modify /issues/search/untriaged to accept categories

This is a temporary workaround of GitHub's lack of sending
Link headers to the Issues API filtered by label. They have been
contacted and confirmed they are tracking it via an internal issue.
Issue #420 - Fetch named label filters from /issues/search/ endpoint

"closed" remains at /issues/category because requesting issues by
issue state correctly returns Link headers.