Add option to recursively process sub directories of sourcePath

[elgordino]

Adds a recursive option that, when set, will traverse the directory structure looking for images to process.

Also adds a warning if a sourceDir includes a directory but recursive is not set.

[simonihmig]

Looks good, thanks a lot @elgordino!
And sorry for the late responsive! This slipped through...

I wonder though if we really need the recursive option, or if we should rather always do a recursive walk? Like is there a use case where you have subfolders in your image folder, but don't want them to be processed?

@andreasschacht thoughts?

[simonihmig]

Sorry for keeping this in limbo for so long. Finally I have reserved some time to work on this addon, and cut a new release asap!