Renamed `WEBVTT_SKIP_CUESETTING` to `WEBVTT_NEXT_CUESETTING`

`NEXT` seemed more appropriate than `SKIP` while I was rebasing my
`split-parser` branch against `dev`. So it's just a quick rename to
keep an extra file change out of my pull request.
Merge pull request #162 from caitp/rename-enum

Renamed `WEBVTT_SKIP_CUESETTING` to `WEBVTT_NEXT_CUESETTING`