Convert NULL byte to U+FFFD (utf8 {EF,BF,BD}) in parser 

Spec asks us to do this:
```
The WebVTT parser algorithm is as follows:

Let input be the string being parsed, after conversion to Unicode, and with the following transformations applied:

Replace all U+0000 NULL characters by U+FFFD REPLACEMENT CHARACTERs.
```
We can't really do this for the entire file, but doing this during `webvtt_string_getline` should cover any situation where this is encountered in a file.

I do believe that the CueText parser will need to perform this as well, so that we can cover cues added via the DOM api or similar.

Perhaps a better way of doing this would be to provide a `string_replace_all` routine, and call this after the line has been read.