Added helper routine to determine if cue is to be skipped.

returns nonzero if the cue has an ID but no cuesettings (the only situation
where we can really have a cue-id and encounter a newline or something).

Might be helpful for debugging eg `p cue_is_incomplete(cue)', might also in
the future make code a bit more readable.

webvtt_bool cue_is_incomplete( const webvtt_cue * )