[enhance] Store unprocessed cuetext in `webvtt_cue`.

Eventually I don't think this string should be touched by the parser, but for the time being it's a simple solution to improve the workings of `read_cuetext()`. Reading a line into `line_buffer`, checking that line, and then appending the line to `cue->body` if necessary. Eventually, I'd like to move all of these onto the general filestructure stack, and the string could be stored in a stack frame on its own instead. However, for the time being it's acceptable to work like this.