Add postalias cmd support in tables

Added the ability to generate /etc/aliases with f.ex postalias from within a table. Example:

         tables: {
           aliases: {
             _type: 'hash',
             _set: 'alias_maps',
             _add: 'alias_database',
             _file: '/etc/aliases',
             _cmd: 'postalias',
             'postmaster:' => 'root',
             'root:' => 'sysadmin@example.com'
           },

It defaults to use postmap.