Wrap columns to 80 characters.

- To do this I had to move the return types and function signatures
onto their own lines in many header files. We already do this in most
.c files. Due to this I've changed all the function declarations to
use this same style.
- Remove trailing whitespace.
Change var name in string.c

- Changed webvtt_string res to be called out as this is more clear
as to what it is actually for.
Make webvtt_string_append comments more clear