Support prediction on a folder

Refs #34 (closed)

The current teklia-dan predict only support prediction on an image through --image. We'd like to be able to process a whole folder.

Add new arguments to the parser:

• --image-dirs, type=Path, in a mutually exclusive group with --image, which is no longer required, (example), no default, path to a directory with images
• --image-extension, type=str, the extension of the images in the folder, defaults to jpg

Refactor the code of the current run function to be able to do something like

def run(...):
    if image:
        process_image(
            image,
            ...
        )
    else:
        for image_name in image_dir.rglob("*.jpg"):
            process_image(
                image_name,
                ...
            )

Please do not load the model multiple times.