Call `parse_cuetext' from `parse_chunk' and remove it from public API. Some other small changes.

Now calling webvtt_parse_cuetext from webvtt_parse_chunk -- parse_cuetext now has the webvtt_parser passed to it, so it may return errors to the application. removed payload from the webvtt_cue struct, as we are using the line_buffer to store the payload, and removing it after parsing. can change this if necessary later.