[IMP] Option to permit custom dump for backup

This improvement is necessary for future migrations.

It permits the user to get the original PostgreSQL custom dump that we
use to have until version 8.0.

@rim-odoo 
Comes to think of it, I can detect it by checking if it's a ```.dump``` and a ZIP mime type at the same time.

Either .zip or .odump sounds good to me. .dump looks nasty though: can be mistaken with a SQL or postgres custom dump.