Deletes temp files after postprocess merge unless -k option is specified

This fixes issue 2108 (https://github.com/rg3/youtube-dl/issues/2108)

For example,

youtube-dl --format='bestvideo+bestaudio' https://www.youtube.com/watch?v=MjQG1s3Isgg

will delete the temp audio-only and video-only DASH files, and

youtube-dl --keep-video --format='bestvideo+bestaudio' https://www.youtube.com/watch?v=MjQG1s3Isgg

will keep those files.