feat: add custom merge option

[alanblins]

Hi,
This is extension to allow the library use custom merge function. By default the library jsonMerge uses Object.assign that has some limitations. Example:
Consider the object below:

var object = {
  'a': [{ 'b': 2 }, { 'd': 4 }]
};
 
var other = {
  'a': [{ 'c': 3 }, { 'e': 5 }]
};

The command Object.assign(object, other) won't merge the keys b and d.

{
 "a":
   [
     {"c":3},
     {"e":5}
   ]
}

The lodash.merge function solves that problem:

const _ = require('lodash');
_.merge(object. other);

Output

{ 'a': [{ 'b': 2, 'c': 3 }, { 'd': 4, 'e': 5 }] }

[karolis-sh]

Hi, yeah legit feature, thanks for the PR!