Add pg_dump --with-{schema|data|statistics} options.

By adding the positive variants of options, in addition to the
negative variants that already exist, users can be explicit about what
pg_dump should produce.

Discussion: https://postgr.es/m/bd0513e4b1ea2b2f2d06f02720c6579711cb62a6.camel@j-davis.com
Reviewed-by: Corey Huinker <corey.huinker@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>