[python] Add option to export "thumbnails" during generation

[Montspy]

We've seen several NFT collections with different image and animation_url fields.
For animated NFTs, usually a GIF is in the image field and the full blown video (MP4 or WebM) is in the animation_url field
For static NFTs, a lower resolution image can be in image and the full resolution in animation_url

I propose we add the option to generate those thumbnails either as a command line argument:
Example: --thumbnail 1280x720 on an animated collection would export GIFs to the collection_name/thumbnails with a resolution of 1280x720
Example: --thumbnail 1200x1200 on static collections would export PNGs to the collection_name/thumbnails with a resolution of 1200x1200

Or as a key/value pair in traits.py/traits.json:

    # No "thumbnails" key/value pair does not generate thumbnails
    "thumbnails": [1280, 720] # Generate thumbnails of that resolution
    "thumbnails": [] # Generate thumbnails of default resolution

[Montspy]

Implemented in #30
Specify thumbnail width (and height, optional) in traits.config["thumbnails"]