Add finish_parsing function.

@caitp or @rillian please review.

**Please Note:** The payload unit tests are now seg faulting because webvtt_parse_cue_text is now being called for the first time due to this update. Before it wasn't being called. Also, it seems to be failing because it expects XFail on CueIdLanguage, but it's getting XPass.

**Edit**
Rebased on Caitlin's fix and it works.

I've added the finish_parsing function. Based on these changes the calling application
will now call this function when the stream has come to an end and the parser should
do it's final operations.