Support for metadata headers

I've started seeing WebVTT files which have a header like this:

```
WEBVTT
Kind: captions
Language: en

00:00:30.460 --> 00:00:35.380
…
```

If I'm reading http://dev.w3.org/html5/webvtt/#webvtt-file-structure correctly, this is a valid file with metadata headers but the validator currently reports an error.
