Use 'webvtt_parse_cuesetting' for 'vertical' cuesettings

I've also updated the csvertical_unittest test set by using the helper
function 'expectDefaultVerticalSetting()' rather than explicitly
asserting individual vertical settings. I've also changed some
assertions that are not really fatal to be expectations instead, such
that they will still cause a failure, but also provide additional
information with subsequent failing tests.
Merge pull request #330 from caitp/vertical-setting

Use 'webvtt_parse_cuesetting' for 'vertical' cuesettings