Feature: "yes to all" for pdftools delete

[petermao]

When this package is used as a library, there are instances where the overwrite query in delete gets in the way. Also, on the command line, it is useful to have a way to get around the overwrite query.

For example:

ls *.pdf  | xargs -I {} pdftools delete {} 1

does not remove the first page of every pdf in the cwd. With this PR in place,

ls *.pdf  | xargs -I {} pdftools -f delete {} 1

would do the trick.