Support mongoose array fields

[JCMais]

Fixes https://github.com/lucasbento/create-graphql/issues/81

[codecov[bot]]

Codecov Report

Merging #94 into master will increase coverage by 4.24%. The diff coverage is 95.74%.

@@            Coverage Diff             @@
##           master      #94      +/-   ##
==========================================
+ Coverage   59.01%   63.26%   +4.24%     
==========================================
  Files          13       13              
  Lines         305      343      +38     
==========================================
+ Hits          180      217      +37     
- Misses        125      126       +1

Impacted Files	Coverage Δ
packages/generator/src/utils.js	95.65% <95.74%> (+0.65%)	:arrow_up:

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[JCMais]

@lucasbento the following is now supported:

{
  array1: [String],
  array2: {
    type: [String],
    description: 'Some description'
  },
  array3: [
    {
      type: ObjectId,
      ref: 'AnotherModel',
    },
  ],
  array4: {
    type: [ObjectId],
    ref: 'AnotherModel',
    description: 'Some description'
  }
}

I'm mutating the AST though, so I don't know if you have any problem with that.