Implement Caitlin's suggestions for passing strings.

- I've changed the cue text parser functions to always pass webvtt_string by ref.
- I've changed cuetext token's struct start token data from a pointer to a
non-pointer.
Merge pull request #192 from RickEyre/cait-suggestions

Implement Caitlin's suggestions for passing strings. I'm not sure what the changes to the cssclasses member has done, but it looks well enough