Generalize non-parametric type handling

[kimikage]

This should not be a breaking change. (Oh, I thought I had created this branch on my fork. Sorry.)

Closes #146.

[codecov]

Codecov Report

Merging #149 (32aa8aa) into master (85b8759) will increase coverage by 0.50%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #149      +/-   ##
==========================================
+ Coverage   92.48%   92.99%   +0.50%     
==========================================
  Files           2        2              
  Lines         213      214       +1     
==========================================
+ Hits          197      199       +2     
+ Misses         16       15       -1     

Impacted Files	Coverage Δ
src/ColorVectorSpace.jl	98.51% <100.00%> (+0.50%)	⬆️

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[kimikage]

This has almost no negative effect on the existing codes, so I'll merge this.
However, I do not plan to release it immediately. Probably more work related to issue #155 will follow.