Natively set cookies

[ColinFay]

To work seamlessly, we should be able to set cookies natively through {brochure}.

[ColinFay]

I think there should be two things here:

• A brochure_session cookie, as is currently implemented at https://github.com/ColinFay/brochure/blob/issue-7/R/brochureApp.R#L93

• Allow to add a cookie on a any given page, something like page(href = , content = , cookie = list())