Adaptive mode for yt-transcribe

I added adaptive mode to yt-transcribe, my shell script for transcribing youtube talks: https://github.com/llimllib/yt-transcribe Adaptive mode looks for scene changes, rather than just generating a thumbnail every N seconds. Here's an example, generating a transcript for Steve Klabnik's talk "Steel, Rust and Truth" That talk motivated the work, as I wanted the script to capture all of the slides - it was hard to follow without that. The actual change is here