Allow customization of fields in `maxctrl list` commands

Being able to define the output of maxctrl list servers would be very useful. This has been an often requested feature and being able to extend the output makes it more customizable. Also allowing the fields to be reordered would make it easier to use maxctrl for scripting.

In addition to the existing fields, at least the following fields should be available:

• Replication lag
• Server version