Add String binding construct that takes a char * array and len.

Fixes #167

- I've added a String binding constructor that takes a char * array and a default len
of -1. This way we can just pass literals into the constructors which will simplify
a lot.
- The String binding constructor that takes a byte * array also accepts a default len
of -1 now.
- webvtt_create_string_with_text now finds the len of the byte * array that it is
passed if the len it is passed is -1.