Add more option for custom config [#503]

[to-long]

Add more option for custom config [#503]
"gen-api": "swagger-typescript-api --custom-config ./open-api/config.js

My custom config file:

const primitiveTypeConstructs = _struct => ({
  date: () => 'string',
  'date-time': () => 'string',
  string: {
    date: () => 'string',
    'date-time': () => 'string',
  },
})

module.exports = {
  primitiveTypeConstructs,
}