Add the merge and removeBatch mutators

[lucasveigaf]

Adds two new mutators: merge for merging an array with the existing field array. removeBatch for removing items from the field array given a list of indexes.

See #26

[codecov]

Codecov Report

Merging #27 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #27   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           9     11    +2     
  Lines          84    101   +17     
  Branches       16     19    +3     
=====================================
+ Hits           84    101   +17

Impacted Files	Coverage Δ
src/merge.js	100% <100%> (ø)
src/removeBatch.js	100% <100%> (ø)

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[lucasveigaf]

This could simply be:
array => array && indexes ? array.filter((item, i) => !indexes.includes(i)) : array

Although much simpler, the performance is significantly worse. Not sure what should be the goal here, since probably there won't be hundreds/thousands of item in the field array.

Any thoughts?

[erikras]

merge should be concat to follow the JavaScript nomenclature.

I'm working on this library today, so I'll merge (😆) this PR and rename it.