Declare webvtt_parse_align and webvtt_parse_line in parser_internal.h
Merge pull request #329 from caitp/declare-in-header

Declare webvtt_parse_align and webvtt_parse_line in parser_internal.h
Merge pull request #331 from caitp/fix-string

String API updates, stronger unit tests for updated features
Merge pull request #330 from caitp/vertical-setting

Use 'webvtt_parse_cuesetting' for 'vertical' cuesettings
Use 'webvtt_parse_cuesetting' for 'size' cuesettings

I've also updated the cssize_unittest test set by using the helper
function 'expectDefaultSizeSetting()' 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.