line breaks for webvtt (and srt) now agree better with the specs.

Because the in the webvtt format, line breaks are part of the syntax,
when converting a CaptionSet to a webvtt, we will be converting extra
line breaks into the &nbsp; symbol. This also has the advantage of being
very explicit and visible.

Same for SRT files - they were vulnerable to having their syntax broken by too many line breaks, for which other formats don't have anything against.