* Add the withCredentials configuration key

It enables passing credentials in CORS requests. e.g. Cookies and
Authorization headers.

* Improve withCredentials documentation

* Add unit tests for the withCredentials config

* Update configuration.md

* Update configuration.md

* only set `withCredentials` Fetch flag if the config value is truthy

there are some workarounds in the wild today that involve setting `withCredentials` on `system.fn.fetch` directly. 

this approach avoids mangling those existing workarounds!

* add more test cases

* Update configs-wrap-actions.js

* Update index.js