Add eyesItInstance

[hepiyellow]

Add a eyesItInstance function which creates an eyes delegator with a default config object,
so you don't have to pass config options in every test.
And still allows different test-suits to have different configs (not a global eyes-it config).

import { eyesItInstance } from 'eyes.it';

const eyes = eyesItInstance({width: 1024, height: 768});

eyes.it('should run test 1', async () => {
    // test 1
});

eyes.it('should run test 2', async () => {
    // test 2
});

The underlaying eyes instance is the same eyes singleton that you get when you do:
import eyes from 'eyes.it'