stbt run: --save-video to record a video to disk

This is easier for the user than having to specify a sink-pipeline like:

    tee name=mytee \
    mytee. ! queue leaky=downstream ! vp8enc speed=7 ! webmmux ! \
        filesink location=video.web \
    mytee. ! queue leaky=downstream ! xvimagesink sync=false

The video is in WebM format, which is playable by modern browsers as
part of the HTML5 standard.

On Fedora, vp8enc is provided by gstreamer-plugins-bad-free, and
webmmux is provided by gstreamer-plugins-good.