Add --no-blobs option to pg_dump

Add an option to exclude blobs when running pg_dump.  By default, blobs
are included but this option can be used to exclude them while keeping
the rest of the dump.

Commment updates and regression tests from me.

Author: Guillaume Lelarge
Reviewed-by: Amul Sul
Discussion: https://postgr.es/m/VisenaEmail.48.49926ea6f91dceb6.15355a48249@tc7-visena
