Experimental: `<relative-time>` supports thresholds that are specified by the user

[zaataylor]

These changes enable the <relative-time> element to support user-supplied thresholds that pull from the value of a newly defined data-preferred-threshold attribute. The distinction between this feature and the pre-existing threshold support offered by the element already is that it will theoretically enable per-user specification of preferred thresholds over all <relative-time> elements. This would enable us to support something akin to a user setting allowing a developer to decide how long times show up as relative before they are shown as absolute time representations.