Add support for multiple input types

The base `juxta.sh`-script takes a `.dat`-file with images (and optionally metadata) as input. This should be more flexible, so input can be

* Files with image paths (or URLs) `images.dat`
* Image filenames `image1.jpg` `image2.jpg`
* Glob patterns `myimages/*.jpg`
* Folders `myimages/`
* Any combination of above