For a 0.4.0 release

[ablaom]

#56

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.62500% with 3 lines in your changes missing coverage. Please review.

Project coverage is 95.07%. Comparing base (89227e3) to head (d639290).
Report is 22 commits behind head on master.

Files with missing lines	Patch %	Lines
src/ScikitLearnAPI.jl	80.00%	2 Missing ⚠️
src/macros.jl	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
+ Coverage   87.43%   95.07%   +7.63%     
==========================================
  Files          12       13       +1     
  Lines         207      264      +57     
==========================================
+ Hits          181      251      +70     
+ Misses         26       13      -13     

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