Get image extensions when not present in `--img-name-col` column

The column used to write the filename may not have a file extension. A user might prefer to have images saved with extensions present in the filenames.

Another optional Boolean flag could be used to give the user the chance to specify something like:

  -e, --infer-extension  Infer the appropriate file extension if one is not present in the --img-name-col (default: False)

If this is switched on, some options mentioned in the discussion for #1 for doing this could be:

First, double check that there isn't a valid image extension in the --img-name-col to avoid writing a file with something like image.png.png (if there's an extension present and the user says -e, I can't imagine why they would have done so intentionally).

To detect an extension:

• Use e.g. filetype to detect the appropriate extension based on the data itself and use that when writing the filename.
  or
• Use the URL column to check for an extension and/or Content-Disposition header

Start at the CLI handling for --img-name-col and the code that writes downloaded images. Review the options discussed in issue #1 and its linked pull request comments, including filetype detection and URL or Content-Disposition checks. Done means an optional --infer-extension flag avoids duplicate extensions and writes an inferred extension when none is present.