Add credentials_cookie as valid option

[ebuzzz]

Configures the JS API to write a secure cookie on your application domain by adding credentials_cookie: true to the framework initialization script tag:

https://developer-programs.linkedin.com/documents/exchange-jsapi-tokens-rest-api-oauth-tokens

Adding the option to the config map with default value false should be enough.

[ebuzzz]

Thanks for fetching the pull. Is it possible to make a new release of the library, so I can include your repo in my project?