Allow enhancing standard header elements

[pd95]

As described in #47 a common problem is that the standard header created by Ignite has to be enhanced with some additional elements.

With this PR I propose the enhancement of the current initializer init(page:,in:) to take an additional parameter which allows appending more elements to Head:

init(for page: Page, in context: PublishingContext, @HeadElementBuilder additionalItems: () -> [HeadElement] = {[]})

This allows enhancing the standard header with as shown below:

Head(for: page, in: context) {
    // Additional headers
    MetaLink(href: "/css/custom.css", rel: "stylesheet")
    Script(file: "/js/custom.js")
}

[pd95]

Additional to above description, I've moved the creation of the "standard set of headers" to Head.standardHeaders(for:, in:) for easier re-use. Additionally it will be easier to write tests if we don't have to duplicate the elements.

[twostraws]

Very nice – thank you!