Support --format=plain option for pg_dump

[fblackburn1]

I would like to have an option to choose how pg_dump will format the database (i.e. plain vs custom) I could make a PR to support it, but I need to know Would you interested to have this kind of option? What do you think of --dump-format (plain, custom directory, tar) and --dump-compress (0..9) ? Or do you prefer something more generic to pass any pg_dump options to the command?