Add pause/resume functionality

Make it possible to pause an active download by hitting SPACE. This works very well when just downloading a file. However, when a video player is connected then it obviously does not as it just deselects the files. To actually stop downloading you would have to stop the video in the player or close it entirely.

@mafintosh what do you think? Is this desirable behaviour or should the stream also be closed then?

Oh, this would implement #50