Allow displayKey to be a function

[jharding]

This will provide more flexibility as the display value of a suggestion object can be formed based on multiple properties of the object, not just one.

[vic]

Yes please 👍

[hoofstephan]

Many thanks for this update. Very useful, especially when using templates incorporating multiple datum properties. Users expect 'what you see is what you get' behavior. Before this (supplying a function) was possible, I created a workaround by adding an extra property in the JSON source file (concat of some other properties), only to reflect the template output. Now I can remove this overhead.