Support SameSite attribute

[Andrew Welch]

For security reasons, it would be nice to be able to set the SameSite attribute on a cookie as per:

https://www.sjoerdlangkemper.nl/2016/04/14/preventing-csrf-with-samesite-cookie-attribute/

https://sites.google.com/a/chromium.org/dev/Home/chromium-security/ssca

https://www.filamentgroup.com/lab/spectre-js-cookie.html

[Alex Chao]

You can write the below code directly:

import { setCookie } from 'tiny-cookie';

setCookie('foo', 'Foo', { SameSite: 'strict' });

I'll update README.md for SameSite attribute.

Thank you for reminding me this.

[Alex Chao]

Updated readme 443b2f8