Add tests/util/cue2json.js for creating test-ready JSON from parsed cues

Looking at Rick's new test harness, it seemed to me like it would be nice if you didn't have to hand-write that JSON.  This adds a node script that you can use to get JSON output suitable for a test run.  You use it like this:

```
$ ./tests/util/cue2json.js some-file.vtt > some-file.json
```
Hopefully this will let Rick go faster.

Rick, can you give this a try and land if you're happy with it?  I can fix if you hit issues.