code: function makeOptionsSchema(additionalProperties) {
    const base =  {
      'type': 'object',
      'properties': {
        'commonjs': { 'type': 'boolean' },
        'amd': { 'type': 'boolean' },
        'esmodule': { 'type': 'boolean' },
        'ignore': {
          'type': 'array',
          'minItems': 1,
          'items': { 'type': 'string' },
          'uniqueItems': true,
        },
      },
      'additionalProperties': false,
    }
  
    if (additionalProperties){
      for (let key in additionalProperties) {
        base.properties[key] = additionalProperties[key]
      }
    }
  
    return base
  }
docs: make an options schema for the module visitor, optionally
  adding extra fields.