Don’t use a default setting for 'order'

Don’t use a default setting for 'order'

This will make i3status depend on a useful configuration file (so
/etc/i3status.conf has to be installed) to actually output anything.

However, it fixes a very common error: people edit their i3status.conf and
don’t get that the first order directive has to use =, not +=. Since order is
now empty by default, you can use += everywhere.