Define method for colorview instead of converting to Matrix{<:Color}

[giordano]

Fix #11.

[coveralls]

Pull Request Test Coverage Report for Build 45

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+8.7%) to 94.444%

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Totals
Change from base Build 20:	8.7%
Covered Lines:	17
Relevant Lines:	18

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💛 - Coveralls

[codecov-io]

Codecov Report

Merging #12 into master will increase coverage by 8.63%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #12      +/-   ##
=======================================
+ Coverage   86.36%   95%   +8.63%     
=======================================
  Files           3     3              
  Lines          22    20       -2     
=======================================
  Hits           19    19              
+ Misses          3     1       -2

Impacted Files	Coverage Δ
src/AstroImages.jl	100% <100%> (+5.88%)	⬆️
src/plot-recipes.jl	100% <0%> (+25%)	⬆️

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