Add io::WebcamCapture via gstreamer

[edgarriba]

• implements a simple gstreamer pipeline to capture frames from a webcam
• the code lives in webcam.rs as WebcamCapture with its own builder

[Wumpf]

basic structure lgtm given my little understanding of this domain 😄
the error handling and the lack of easy cancel & memory control makes me a bit uneasy though

[edgarriba]

@Wumpf @gau-nernst would be great if you could try the examples in your computer