Wrap trees for better display

[ablaom]

Closes #45

using MLJBase, MLJDecisionTreeInterface
mach = machine(DecisionTreeClassifier(), (@load_iris)...) |> fit!

julia> tree = fitted_params(mach).tree
petal_width < 0.8
├─ setosa (50/50)
└─ petal_width < 1.75
   ├─ petal_length < 4.95
   │  ├─ petal_width < 1.65
   │  │  ├─ versicolor (47/47)
   │  │  └─ virginica (1/1)
   │  └─ petal_width < 1.55
   │     ├─ virginica (3/3)
   │     └─ sepal_length < 6.95
   │        ├─ versicolor (2/2)
   │        └─ virginica (1/1)
   └─ petal_length < 4.85
      ├─ sepal_width < 3.1
      │  ├─ virginica (2/2)
      │  └─ versicolor (1/1)
      └─ virginica (43/43)

# using Plots, TreeRecipe
# plot(tree)

[ablaom]

@roland-KA Would you have time to give this a quick once over the next week?

[codecov-commenter]

Codecov Report

Merging #50 (ae29ef4) into dev (ebd0c7c) will decrease coverage by 0.56%.
The diff coverage is 94.44%.

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@@            Coverage Diff             @@
##              dev      #50      +/-   ##
==========================================
- Coverage   98.40%   97.84%   -0.56%     
==========================================
  Files           1        1              
  Lines         125      139      +14     
==========================================
+ Hits          123      136      +13     
- Misses          2        3       +1     

Impacted Files	Coverage Δ
src/MLJDecisionTreeInterface.jl	97.84% <94.44%> (-0.56%)	⬇️

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[roland-KA]

This looks quite good to me ... and it is a real improvement with respect to usability; good idea!

[ablaom]

@roland-KA Thanks for your review.