Suggest to add `::cue`, `:past`, `:future` and `::cue-region` support. This is from WebVTT spec[1]. And major browser all supports WebVTT now[2]. Spec[3]. [1]:https://developer.mozilla.org/en-US/docs/Web/API/WebVTT_API#Styling_WebTT_cues [2]:http://caniuse.com/#feat=webvtt [3]:https://w3c.github.io/webvtt/#css-extensions-processing-model
