Allow mutation of image or index media types

[jonjohnsonjr]

Fixes #491

For reasons that are fun to explain, I need this for ko-build/ko#38 to work with GCR, as well 😄

[codecov-io]

Codecov Report

Merging #624 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #624      +/-   ##
==========================================
+ Coverage   76.27%   76.37%   +0.09%     
==========================================
  Files         101      101              
  Lines        4397     4415      +18     
==========================================
+ Hits         3354     3372      +18     
  Misses        633      633              
  Partials      410      410

Impacted Files	Coverage Δ
pkg/v1/types/types.go	100% <ø> (ø)	⬆️
pkg/v1/mutate/mutate.go	74.56% <100%> (+1.23%)	⬆️
pkg/v1/mutate/index.go	76.81% <100%> (+1.81%)	⬆️
pkg/v1/mutate/image.go	68.75% <100%> (+1.27%)	⬆️

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