WkHtmlToPdf now supports header/footer (simple)

Added helpers to facilitate easy header/footer and from View/Layout configuration of WkHtmlToPdf creation.
We certainly could abstract further away from the `WkHtmlToPdfEngine` options and translate into their options... but I wanted to keep it as close to their syntax as possible, so we could use their documentation...

There are now helpers for `View->pdfHeader('spacing', 8)` and `View->pdfHeader('left', $title_for_layout)` but I found myself quickly going back to just putting into one big array.

If you still think so, we could translate header+children pretty easily if we wanted to.