Supports headers and omitting hours

[proudust]

Allow text header

4.1. WebVTT file structure

A WebVTT file body consists of the following components, in the following order:

1. An optional U+FEFF BYTE ORDER MARK (BOM) character.
2. The string "WEBVTT".
3. Optionally, either a U+0020 SPACE character or a U+0009 CHARACTER TABULATION (tab) character followed by any number of characters that are not U+000A LINE FEED (LF) or U+000D CARRIAGE RETURN (CR) characters.

Optional if the hours is zero

4.1. WebVTT file structure

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 (:)

[spaceribs]

Looks great! pulling this down and checking it.