WebVTT decoder doesn't handle html escapes

[RiCON]

WebVTT spec specifies a dozen HTML escapes that should be handled, including '>', '<' and '&'. These aren't converted back to the proper characters.

FFmpeg version:

% ffmpeg -i htmlescapes.vtt out.srt
ffmpeg version N-75818-g8135b1e Copyright (c) 2000-2015 the FFmpeg developers
  built with gcc 5.2.0 (Rev4, Built by MSYS2 project)

Attached is an example vtt file, result with this build and proper result.
Examples of where these html escapes are used can be found by getting the subtitles from any video in Comedy Central's site using something like youtube-dl. Example:

% youtube-dl --all-subs "http://www.cc.com/video-clips/52dpzm/the-daily-show-with-trevor-noah-terrible--unending-national-tragedies"

[RiCON]

Resulting .srt from ffmpeg

[RiCON]

Proper .srt with escapes converted

[RiCON]

Example of WebVTT with escapes as downloaded using youtube-dl

[RiCON]

Forgot to link to the spec: ​http://dev.w3.org/html5/webvtt/#dfn-webvtt-cue-amp-escape

[RiCON]

Added more test tags and replacements

[RiCON]

Fixed in 53886d6955134be8acc26f336bdf068fd970669d.