Add border property to PageBreak

[stenci]

Sometimes the PDF contains horizontal grey lines, so the PageBreak lines get confused with the ones that are part of the PDF.

This new border property allows to change the default style to something different form the rest of the PDF. It has no effect on the generated PDF.

In one of my PDFs I'm using:

self.add_component(TruckPDF(self.truck))
for crate in self.truck.crates:
  self.add_component(PageBreak(border='1px dashed red'))
  self.add_component(CratePDF(crate))

close #142

[s-cork]

lgtm (looks good to me) - some code suggestions.

Edit - and can we update CHANGELOG.md?

[s-cork]

@stenci - are you happy to resolve the suggestions above with another commit?
(I'm also happy to resolve them).

[s-cork]

Perfect - thanks for fixing #142 - merging