Make PDFs additive and modular

[Donnype]

A nice feature would be the option to do:

import pydfy.models as pf

pdf = pf.PDF(...)
pdf2 = pf.PDF(...)

new_pdf = pdf + pdf2  # Although a more explicit pdf.merge(pdf2) makes sense to add as well
new_pdf = new_pdf.add(pf.Number(2))  # Accepting either a row or a component
new_pdf.render()  # get both PDFs merged with a PageBreak, and the Number component at the end

Because this would allow users to create larger PDFs in a modular way and pass around well-defined (ideally immutable) PDF objects. Perhaps a merge method should have an optional boolean pagebreak argument defaulting to True. Both these methods should be immutable.