The whole-file ASS parser is now a simpler version that reads line-by-line from a stream.

- Added ASS.fromUrl and ASS.fromStream that can load an ASS from a URL and from a libjass.parser.Stream respectively.
- ASS.fromString, ASS.fromUrl and ASS.fromStream all return a Promise<ASS>. This is a breaking change for ASS.fromString
- Added ASS.addStyle that can be used to dynamically add Style objects to an ASS object.
Changed SRT parser to a stream parser as well.
Updated README for the new API.
Stream parsers for ASS and SRT.