Add Support for WEBP images

[Lonniebiz]

WEBP is a standard image format, but Viewnior doesn't display them.

Browser Support of WEBP images:
https://caniuse.com/webp

WEBP Info:
https://developers.google.com/speed/webp

[ncmprhnsbl]

#20
ie. get webp-pixbuf-loader
the only other thing would be to edit viewnior.desktop line: MimeTypes and add image/webp

[hellosiyan]

Duplicate of #20
As mentioned above, there are loader modules for GDK-PixBuf that add support for "exotic" file formats.