Merge overlay.png with media.mp4

[king-millez]

Snaps with text and stickers don't include said graphics in the video file, instead they're stored in an image called overlay.png, and displayed over the top by the browser/app.

We could have an option like --merge-overlay which would use something like ffmpeg or avconv to put the image over the top of the media.mp4 file and export it as a new file.

[king-millez]

I believe @hack-green wanted to work on some of this stuff. how's that going so far?