Add MLJBalancing to MLJ and add class imbalance docs

[ablaom]

This PR needs:

• Support Julia 1.6 MLJBalancing.jl#20

This PR should ideally wait for:

• Problem with docstring for BalancedModel MLJBalancing.jl#19

This PR closes: #1063 #1062

[codecov-commenter]

Codecov Report

Merging #1064 (fcf44d2) into dev (7b32802) will not change coverage.
The diff coverage is n/a.

❗ Current head fcf44d2 differs from pull request most recent head 5063acf. Consider uploading reports for the commit 5063acf to get more accurate results

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  Coverage   57.89%   57.89%           
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  Files           2        2           
  Lines          38       38           
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  Hits           22       22           
  Misses         16       16           

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

[ablaom]

@EssamWisam Be good if you can comment on the new manual page, docs/src/correcting_class_imbalance.md .

(BTW, The docs don't get generated until the master <- dev PR)

[EssamWisam]

Thanks for the marvelous effort @ablaom. It's honoring to see Imbalance being exposed in MLJ docs.

[ablaom]

@EssamWisam Thanks for the review.