Revert default metric to DE_2000

[adrhill]

Reverts #87. This is slower, but the output is of much higher quality:

white = RGB{Float32}(1, 1, 1)
yellow = RGB{Float32}(1, 1, 0)
green = RGB{Float32}(0, 0.5, 0)
orange = RGB{Float32}(1, 0.5, 0)
red = RGB{Float32}(1, 0, 0)
blue = RGB{Float32}(0, 0, 1)

rubiks_colors = [white, yellow, green, orange, red, blue]

• dither(img, rubiks_colors; metric=DE_AB())
  
• dither(img, rubiks_colors; metric=DE_2000())
  

[codecov]

Codecov Report

Merging #92 (c72ec83) into master (09d158d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #92   +/-   ##
=======================================
  Coverage   97.95%   97.95%           
=======================================
  Files          12       12           
  Lines         293      293           
=======================================
  Hits          287      287           
  Misses          6        6           

Impacted Files	Coverage Δ
src/api/color.jl	90.00% <ø> (ø)

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