add a parameter to include credentials

[steventill]

Description

Add a property that when set includes the credentials
http://www.html5rocks.com/en/tutorials/service-worker/introduction/
// see the section on the defaults of fetch

Expected outcome

when set to true, passes sets the credentials flag and uses that when fetching resources

Actual outcome

does not use the credentials (Note: also ignores the mode: 'no-cors' attribute when fetching)

Steps to reproduce

Browsers Affected

Feature request !-- all browsers