Add button to upgrade youtube-dl

[Kagami]

Need to just spawn youtube-dl --update.

[Hrxn]

From README.md

NOTE: wybm will use system (i.e. in the PATH) youtube-dl, ffmpeg and mkvinfo if available.

Doesn't that mean that wybm will use the latest version of youtube-dl, if present in PATH?

[Kagami]

Yea. Just for the people who don'w know how/want to deal with PATH and CLI utilities. It's not that user-friendly on Windows.