Add 'basebackup_to_shell' contrib module.

As a demonstration of the sort of thing that can be done by adding a
custom backup target, this defines a 'shell' target which executes a
command defined by the system administrator. The command is executed
once for each tar archive generate by the backup and once for the
backup manifest, if any. Each time the command is executed, it
receives the contents of th file for which it is executed via standard
input.

The configured command can use %f to refer to the name of the archive
(e.g. base.tar, $TABLESPACE_OID.tar, backup_manifest) and %d to refer
to the target detail (pg_basebackup --target shell:DETAIL). A target
detail is required if %d appears in the configured command and
forbidden if it does not.

Patch by me, reviewed by Abhijit Menon-Sen.

Discussion: http://postgr.es/m/CA+TgmoaqvdT-u3nt+_kkZ7bgDAyqDB0i-+XOMmr5JN2Rd37hxw@mail.gmail.com
