fix shell injection upon calling pdftk

[notevenaperson]

I came across an error like this:

$ pdfoutliner toc.txt --inpdf "(John Smith) IRS.pdf" --outpdf '(John Smith) IRS new.pdf'
sh: 1: Syntax error: "(" unexpected

Again the argument is easily intepreted as a shell command, which easily causes trouble:

$ pdfoutliner toc.txt --inpdf "IRS.pdf" --outpdf '; exit'
Error: Failed to open output file:
   /home/user/
   No output created.
   No output created.

(I should be able to name a file '; exit' if I want to)