add optional callback

[t-bltg]

For JuliaPlots/UnicodePlots.jl#289.

[codecov]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (f5ce72b) compared to base (c988dc2).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head f5ce72b differs from pull request most recent head 0d82f93. Consider uploading reports for the commit 0d82f93 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##            master        #9   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          143       148    +5     
=========================================
+ Hits           143       148    +5     

Impacted Files	Coverage Δ
src/ascii.jl	100.00% <100.00%> (ø)

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