Release v4.0.0

[gsantiago]

• Fixes Add streaming support #6 by introducing the stream interface (parse, stringify and resync are now stream-based functions)
• Add parseSync and stringifySync as synchronous version of parse and stringify
• Add map and filter to manipulate the parse stream
• Update the nodes tree so it can support more types than just a cue
• Refactor the internals by creating the Parser and Formatter classes
• Format types are now "SRT" and "WebVTT" instead of "srt" and "vtt"