Support composite vertex/edge labels

**Is your feature request related to a problem? Please describe.**
The '--display-property' and '--edge-display-property' query magic parameters currently only allow for a single property value to be selected. It would be nice to be able to select and combine several properties for a label.

For example, if a query returns the vertex:

`{T.id: 1, T.label: 'person', 'firstname': 'foo', 'lastname': 'bar'}`

It would nice to be able to pass in something like:

`my_node_labels = ‘{“person”:“firstname+lastname”}’`

And get `foobar` as the final label on the visualized node.