Add WebP support

Hello!

Problem/Motivation

WebP extension is not in the allowed formats list of the module, so images are bypassed and cant be downloaded.

Steps to reproduce

return [
            '#theme' => 'imagecache_external',
            '#uri' => 'https://amp.dev/static/inline-examples/images/mountains.webp',
            '#style_name' => 'thumbnail',
            '#alt' => ''
        ];

Proposed resolution

A patch is joined to this issue, where webp is manually added to the existing lists.
2 files are updated.

Thank you