[NFR] Import pdf as background

[ruudboon]

Would it be possible to add an import function that allows us to import an existing pdf as a page where you can add custom elements?

[kbrock84]

See issue #227
This may be a duplicate.

[nrkn]

I have a client who is heavily committed to pdfkit for their workflow that needed to do this and they ending up doing a workaround where they used some other library to covert the second PDF to svg and then used svg-to-pdfkit to place it - bit clunky, but it works