Support parsing metadata from input

[abstractvector]

In reference to issue #18, this pull request adds support for an options object of the form { meta: true } to be passed through to the parse() method. If specified, any additional lines after the WEBVTT line will be parsed - split on the : separator, trimmed and inserted into a meta object on the response.

If the option is false or not passed in, a meta property is not included in the response. If it is true but there is no metadata available, the response includes meta: null.

I have added several more tests and the entire suite is passing. I have also updated README.md to include usage.

I have not updated the compiler so metadata will not be compiled into a string. This shouldn't be challenging but we're not using the compile functionality of this module.

[osk]

Released in v1.3

[abstractvector]

Great, thanks for the quick merge and release!

[osk]

No problem, thanks for the PR.

[meoyawn]

unfortunately segmenter did not benefit from this change

node-webvtt/lib/segmenter.js

Line 8 in 8dae70d

const parsed = parse(input);