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 * )
Merge pull request #262 from caitp/cue-is-incomplete

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