Added an update command.

The update button forces ytsm to reload the feed file. This is useful in
the event that ytsm-cronjob updates the feed while ytsm is running. It's
basically a more graceful way to do '^C ytsm'.
Add download command; fix typo

The download command is very basic. It just uses youtube-viewer to
download the video to the current working directory. No support for
--ffmpeg-command yet.