Passing all but two 'line' cuesetting tests.

I've integrated the webvtt_parse_line() routine from my large patch, and am
calling it in the same fashion as I am calling the parse_align routine, so we
get some uniform results from the two tests.

There are still two tests failing, which stems from not having a standard on
which errors to return and which errors to expect. These two other tests will
also require unit test changes, and will be corrected in a followup pull
request.