Adding gstreamer based plugin

Gstreamer based plugin utilizing gstreamer elements to capture
screen from X, convert and encode into h264/h265/vp8/vp9/mjpeg stream
Configure with --enable-gst-plugin, will be built as a separate plugin.

The plugin was made for testing purposes, it was mainly tested with
the x264enc (h264 is the defualt codec) encoder.
To choose codec type use: '-c gst.codec=<h264/h265/vp8/vp9/mjpeg>'
To specify a certain plugin use: '-c gst.encoder=<plugin name>' in
addition to its matching codec type (gst.codec).

Signed-off-by: Snir Sheriber <ssheribe@redhat.com>
Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Acked-by: Frediano Ziglio <fziglio@redhat.com>