Allow for using consumers jest config

[AdilBhayani]

This PR allows the the consumer of this repo to specify a jest.config.js file in the root of their own repo. If it is present then it will be used. Otherwise we fallback to the jest.config.js which is present in this package.

I've tested this using yalc and https://github.com/AdilBhayani/Date-time-convertor.

[zzarcon]

we have a similar logic in src/utils/createFile.ts I'd recommend extracting that into own util and not using the sync version:

import { promisify } from 'util';
import {exists} from 'fs'

const existFile = (path: string) => {
  return promisify(fs.exists)(path);
}

[zzarcon]

Also it will be nice to have a util to resolve user (or upper) path

import {consumerPath} from '../utils/'

const upperJestPath = consumerPath('jest.config.js');