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.
Merge pull request #46 from vladiibine/master

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