Enable pg_dump format option.

The format option of pg_dump enables tar and custom archive file formats in
addition to the default plain-text file containing SQL commands.

When either the tar or custom format are selected the behaviour of database=all is changed to no longer dump a single file via pg_dumpall. Instead pg_dumpall
is called once to export the "global" data (roles & tablespaces) and then
pg_dump is called once for each non-template table in the database.

To support the GZIP and GZIP_OPTS variables in backupninja and to give the
default --rsyncable gzip compression flag a chance at working on a PostgreSQL
backup, the custom output is forced to not use compression. Instead compression
is done via a pipe to gzip. Hopefully this benefits rsync and rdiff-backup
style backups for reduced backup and storage costs that outweigh the
restoration ones.