Add argument "--file-type" to be able to choose between image or video

[pabera]

I had a use case where I wanted to copy images and videos into separate output paths. I therefore added an argument called "file-type". When present with a value of either image or video, it only looks at files of this type and ignores the other type.

phockup /input /output --file-type image

The above command would only copy/move images and ignore videos.

The log output for an ignored files looks like this

[INFO] - /input/my-video.mp4 => skipped, file is 'video' but looking for 'image'

If you wish to merge it, I suggest squashing it before ;-)
Looking forward to your feedback

[pabera]

Implements more or less #80