Add support for collection to fieldExtractor

[vlaurin]

Resolves #130

Example:

import {fieldExtractor} from '@quickcase/node-toolkit';

const aCase = {
  id: '1234123412341234',
  state: 'Open',
  data: {
    collection3: [
      {id: '123', value: {field2: 'value3'}},
      {id: '456', value: {field2: 'value4'}},
    ]
  }
};

// Prepare fields for extraction
const fields = fieldExtractor(aCase);

// Extract from collections
const value3 = fields('collection3[0].field2'); // By index
const value4 = fields('collection3[id:456].field2'); // By id