Validator doesn't notice if the hours field in the timestamp (if present) has only one digit

[yclicc]

Related to #17 but this time for the hours field. The WebVTT format requires two digits at least if the hour field is present (see https://developer.mozilla.org/en-US/docs/Web/API/WebVTT_API#cue_timings ). This caused issues for me downstream with the AblePlayer video viewer as it silently failed to deal with captions after the hour mark when this wasn't correct.

[davemevans]

Whilst the format does require that cue timings must have two or more digits if hours are in use (https://w3c.github.io/webvtt/#webvtt-timestamp), the algorithm specified for parsing timestamps explicitly does not validate that hours has two or more digits (https://www.w3.org/TR/webvtt1/#collect-a-webvtt-timestamp).

As stated in #17 (comment), webvtt.js implements that timestamp parsing algorithm as written in the spec, with the validator based on that.