Fixes #295 - Store unprocessed cuetext in `webvtt_cue::body`

Accessible from C++ wrappers with WebVTT::Cue::body()

Initialized with empty_string in webvtt_create_cue()

Released in webvtt_release_cue()
Merge pull request #295 from caitp/issue-295

[enhance] Store unprocessed cuetext in `webvtt_cue`.