Add webp support

[saimn]

Ref #429. Thanks to @Mousketeer for the example images.

[codecov]

Codecov Report

Merging #433 (883b523) into main (fee8a3c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #433   +/-   ##
=======================================
  Coverage   86.52%   86.52%           
=======================================
  Files          22       22           
  Lines        1833     1833           
=======================================
  Hits         1586     1586           
  Misses        247      247           

Impacted Files	Coverage Δ
sigal/settings.py	97.87% <ø> (ø)
sigal/video.py	90.75% <0.00%> (-0.85%)	⬇️
sigal/image.py	91.66% <0.00%> (+0.43%)	⬆️

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