Hammer CLI

Output fields can currently be only added to the end of the field list. Hammer's output DSL should provide interface for removing fields and adding nested fields. This functionality is important for plugins to allow them to better structure the output and place fields into logical structure.

It will probably require adding some kind of unique identifiers to fields that would be used for addressing (it could be the field key by default with possibility to override). Such identifiers could be also later reused for selection of fields in the output (see: http://projects.theforeman.org/issues/19135).