Adding pipe support to pg_dump and pg_restore

Hello,

I recently wanted a way to encrypt/decrypt backups while still
utilizing the parallel dump/restore functionality. I couldn't see a
way to do this so I experimented a bit with the directory backup
format. If there's in fact already a way to do this, please tell me
now :-)

The idea is to add a --pipe option to pg_dump / pg_restore where you
can specify a custom shell command that is used to write / read each
.dat-file. Usage examples include encryption with pgp and/or custom
compression pipelines. %p in the command is expanded to the path to
write to / read from. The pipe command is not applied to the toc.

The current version is attached. Could something like this be
acceptable for inclusion?