feat - Allow advanced merging through mergeWithRules

[bebraw]

See the readme for the full example.

Closes #146 and #149.

[codecov-commenter]

Codecov Report

Merging #150 into master will decrease coverage by 0.72%.
The diff coverage is 97.93%.

@@            Coverage Diff             @@
##           master     #150      +/-   ##
==========================================
- Coverage   99.15%   98.43%   -0.73%     
==========================================
  Files           5        6       +1     
  Lines         119      192      +73     
  Branches       34       52      +18     
==========================================
+ Hits          118      189      +71     
- Misses          1        3       +2     

Impacted Files	Coverage Δ
src/index.ts	98.36% <97.59%> (-1.64%)	⬇️
src/join-arrays.ts	96.42% <100.00%> (-0.80%)	⬇️
src/types.ts	100.00% <100.00%> (ø)
src/utils.ts	100.00% <100.00%> (ø)

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