Add support for optional cmd in table params (e.g. postalias)

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'
       },