add 2-args show & tests

[dkarrasch]

Fixes #260 (which has been partly fixed already).

[codecov-io]

Codecov Report

Merging #296 into master will increase coverage by 0.14%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #296      +/-   ##
==========================================
+ Coverage   47.56%   47.71%   +0.14%     
==========================================
  Files          21       21              
  Lines        1068     1071       +3     
==========================================
+ Hits          508      511       +3     
  Misses        560      560

Impacted Files	Coverage Δ
src/io.jl	67.12% <100%> (+1.4%)	⬆️

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