Fix timestamps and strip extraneous whitespace in WebVTT output (#219)

* 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.

whisper/utils.py

@@ -40,7 +40,7 @@ def format_timestamp(seconds: float, always_include_hours: bool = False, decimal
     seconds = milliseconds // 1_000
     milliseconds -= seconds * 1_000
 
-    hours_marker = f"{hours}:" if always_include_hours or hours > 0 else ""
+    hours_marker = f"{hours:02d}:" if always_include_hours or hours > 0 else ""
     return f"{hours_marker}{minutes:02d}:{seconds:02d}{decimal_marker}{milliseconds:03d}"
 
 
@@ -54,7 +54,7 @@ def write_vtt(transcript: Iterator[dict], file: TextIO):
     for segment in transcript:
         print(
             f"{format_timestamp(segment['start'])} --> {format_timestamp(segment['end'])}\n"
-            f"{segment['text'].replace('-->', '->')}\n",
+            f"{segment['text'].strip().replace('-->', '->')}\n",
             file=file,
             flush=True,
         )