add PdfPageEvent.onBeforePageStart

[gered]

This event is useful to do per-page adjustments to page size and margins, etc. These types of page properties cannot be changed in onStartPage or onEndPage for the current page... instead, they would take effect in the next page.

Can be used like other page-events, by using the PdfPageEventHelper class, e.g.:

(proxy [cljpdf.text.pdf.PdfPageEventHelper] []
  (onBeforeStartPage [writer doc]
    ; do stuff here
    ))

One slight oddity, is that this new event will fire before the onOpenDocument event for the first page of the document. onOpenDocument fires too late unfortunately and I was worried about potentially breaking backward-compatibility for anyone that might be making heavy use of page-events with clj-pdf.

[yogthos]

👍

[coveralls]

Coverage remained the same at 48.842% when pulling 9c27450 on before-start-page-event into 418b34b on master.