Simplify multi-byte comparisons in cuetext parser.

Partial fix for #204. We still need to find a way to easily compare multi-byte encodings that can't be represented in a literal char array such as the right-to-left mark.

- I've simplified the multi-byte comparisons in the cue text parser by creating a
webvtt_string_compare function.
- I've also removed the definitions for escape sequences and just used literal
char * where I can. This should be fine since our default encoding is UTF8 which
is compatible with literal char arrays.