Rename/merge Time-relative pseudo-selectors

[foolip]

This feature is for the :past and :future pseudo-classes.

They are really part of WebVTT, and should either be renamed to a "webvtt-past-future" feature, or perhaps just merged into the main WebVTT features.

This depends on #91.

[foolip]

Note also the use of "pseudo-selectors" instead of "pseudo-class", the only instance of this in web-features. It even appears in the feature ID, making a rename "necessary" IMHO.