This commit adds the ability to grab the transcript
of a YouTube video with timestamps. The timestamps
are formatted as HH:MM:SS and are prepended to
each line of the transcript. The feature is enabled
by the new `--transcript-with-timestamps` flag,
so it's similar to the existing `--transcript` flag.
Example future use-case:
Providing summary of a video that includes timestamps
for quick navigation to specific parts of the video.
