Allow one arg version of violin and boxplot

[piever]

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[codecov-io]

Codecov Report

Merging #66 into master will increase coverage by 0.14%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master      #66      +/-   ##
==========================================
+ Coverage   66.59%   66.73%   +0.14%     
==========================================
  Files          16       16              
  Lines         452      460       +8     
==========================================
+ Hits          301      307       +6     
- Misses        151      153       +2

Impacted Files	Coverage Δ
src/recipes/boxplot.jl	89.47% <100%> (+0.18%)	⬆️
src/recipes/violin.jl	93.75% <75%> (-6.25%)	⬇️

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[sethaxen]

#107 also contains an implementation of a single arg version. Should we keep that or go with this one?