* Use two-digit hours in WebVTT timestamps
Per the WebVTT specification [0]:
> A WebVTT timestamp consists of the following components, in the given
> order:
>
> 1. Optionally (required if hours is non-zero):
> 1. Two or more ASCII digits, representing the hours as a base ten
> integer.
> 2. A U+003A COLON character (:)
YouTube won’t accept timestamps containing single-digit hours.
[0] https://www.w3.org/TR/webvtt1/#webvtt-timestamp
* Strip segment text in WebVTT output
We already do this for plain text and SubRip output, so we should do it
for WebVTT too.
