Feature Request: new option to select image file type

[caigner]

Hello!

metapixel is a fine piece of software. When it runs it tries every file it finds in the given directories.
I have jpg, xcf, raw, and xmp files in my image folder (from gimp and darktable)

That causes a lot of errors.

It would be nice to be able to restrict metapixel to scan only for certain image types like jpg, png, gif, tif...
Maybe an option like -t (--type) which accepts a comma seperated list of image types.

Example:
metapixel --metapixel -t jpg,png -l ~/pictures in.jpg out.jpg