Fixes #165, #139 - No longer pushing ptrs to local structs on stack

Stack management is also made simpler by removing the ambiguity of
whether or not a webvtt_string in `v.text` needs to be released and
deleted, or simply released.

Now that the member is not a pointer, it is safe to simply release it.
Fixes #165 - Correct style of patched lines

Accidentally had a tab indented line (src/libwebvtt/parser.c:915). Have
corrected this by replacing the tab with ts=2 expandtab indentation, as per the
library style guide.
Merge pull request #165 from caitp/issue-165

Strings on parser state stack should not be pointers to strings