"--only-new" for fetching only the new comments from the last time #49

This argument makes the script check the output file first (if it exists) to find the timestamp of the latest comment and stop fetching new comments once it reaches that timestamp.

I believe this feature would save time and bandwidth for videos with 1000s of comments. Also less DDoS to the Youtube API :-)

One issue to come into mind is, if I'm not wrong, this feature will miss new replies that are made to old comments. If there is no easy fix for this, it's fine for me.