Support dots in identifiers

[Eternal-Rise]

#19

• tests and linter show no problems (npm t)
• tests are added/updated for bug fixes and new features
• code is properly formatted (npm run fmt)
• description of changes is added in CHANGELOG.md
• update .d.ts typings

Basic solution for current test case

[Eternal-Rise]

@tshemsedinov , fixed. While testing, found a bug with metatests. I'll create an issue.
metarhia/metatests#269

[tshemsedinov]

Need more test cases

[Eternal-Rise]

@tshemsedinov , added a few and I have a question. Should we return an object in that case?

metatests.test('Read deep non-table identifiers', async (test) => {
  const sheet = new Sheet();

  sheet.cells['item.price.usd'] = 100;
  sheet.cells['item.price.uah'] = 200;
  test.strictSame(sheet.values['item.price'], { usd: 100, uah: 200 });

  test.end();
});

And I broke metatests again. That test case passed for some reason. And if we place any check after it, it will pass too.

test.strictSame(sheet.values['item.price'], { usd: 100, uah: 200 });  // -> not fails
test.strictSame(1, 0) // -> obviously, should fail, but it isn't

Created an issue metarhia/metatests#270