Add default configuration file

[camlafit]

Hello

In try to provide a package version, could be nice to allow configuration.

Following debian logic could be an /etc/default/package file as default configuration.
Followign FHS could be an /etc/shrinkpdf file to overwrite configuration

I've tried to do it with 86bad6d
But if we want not be debian centric, should be a test on /etc/shrinkdpf if present.