Revert "Support for categories using the YAML tag 'categories'"

This reverts commit d3dfb84016a036b70d6797263b7939440378263e.
Revert "Revert "Support for categories using the YAML tag 'categories'""

This reverts commit 9f9fec127bb843e74a1019ddfed31e590ff9e02b.
Revert "Support for categories using the YAML tag 'categories'"

This reverts commit d3dfb84016a036b70d6797263b7939440378263e.