WkHtmlToPdf now supports header/footer (simple)

Added helpers to facilitate easy header/footer and from View/Layout configuration of WkHtmlToPdf creation.
Sorry I disagree... if you take a look at [this existing line](https://github.com/FriendsOfCake/CakePdf/blob/master/Pdf/Engine/WkHtmlToPdfEngine.php#L88) it has lower-case `dash-separated` options, because that's what the `WkHtmlToPdfEngine` expects for options...  we could make them `camelCased` and then use `Inflector` to translate, but that seems excessive/unnecessary.