fix: concat arrays in babelOptions

[buschtoens]

What: babelOptions.plugins cannot be used together with fixture tests, as the plugin config gets overridden.

Why: To allow setups like:

pluginTester({
  plugin: myPlugin,
  fixtures: join(__dirname, '__fixtures__'),
  babelOptions: {
    plugins: ['@babel/plugin-syntax-class-properties']
  }
});

// __fixtures__/example/options.json
{
  "foo": "bar"
}

How: Replaced lodash.merge with lodash.mergeWith with the following customizer:

function mergeCustomizer(objValue, srcValue) {
  if (Array.isArray(objValue)) {
    return objValue.concat(srcValue)
  }
  return undefined
}

I found the test setup a bit confusing, but I hope that I added a good test scenario.

[codecov-io]

Codecov Report

Merging #43 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #43   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines         129    132    +3     
  Branches       32     33    +1     
=====================================
+ Hits          129    132    +3

Impacted Files	Coverage Δ
src/index.js	100% <100%> (ø)	⬆️

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[kentcdodds]

@all-contributors please add @buschtoens for code and tests

[kentcdodds]

@all-contributors please add @buschtoens for code and tests