UnicodePlots: png output

[t-bltg]

For JuliaPlots/UnicodePlots.jl#239.

[codecov]

Codecov Report

Merging #4149 (5334937) into master (5872bde) will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #4149      +/-   ##
==========================================
+ Coverage   62.54%   62.55%   +0.01%     
==========================================
  Files          29       29              
  Lines        7315     7333      +18     
==========================================
+ Hits         4575     4587      +12     
- Misses       2740     2746       +6     

Impacted Files	Coverage Δ
src/backends/unicodeplots.jl	48.12% <0.00%> (-5.13%)	⬇️
src/args.jl	73.18% <0.00%> (-0.16%)	⬇️
src/layouts.jl	66.66% <0.00%> (+0.29%)	⬆️
src/backends/gr.jl	88.77% <0.00%> (+0.40%)	⬆️
src/recipes.jl	59.46% <0.00%> (+0.44%)	⬆️
src/axes.jl	85.49% <0.00%> (+0.66%)	⬆️

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