Add support for identifers

[gsantiago]

Currently, this module has not support for identifiers. It would be interesting to add support to identifiers like the examples below:

webvtt

WEBVTT

1 - Some ID here
00:00:22.230 --> 00:00:24.606
This is the first subtitle.

The caption above would be parsed to this:

{
    id: '1 - Some ID here',
    start: 20000,
    end: 24400,
    text: 'This is the first subtitle'
  },

The stringify should support this feature too.

[danielsfirnaciuc]

Hi @gsantiago any news on this little fellow? :)

[gsantiago]

@danielsfirnaciuc nothing yet