Add POSTGRES_SETUP_ARGS for setup configuration

[gsf]

Set the POSTGRES_SETUP_ARGS environment variable to alter configuration
during setup. Useful for settings like "max_wal_size=4GB" and
"fsync=off" when loading initial data. Example pointing at a config
file:

POSTGRES_SETUP_ARGS='-o "-c config_file=/etc/postgresql/setup.conf"'

[gsf]

Tested locally with PostgreSQL 10.

[yosifkit]

Duplicate of #317.