Additional functions in mapping util

jsonpath-ng already provides a decent amount of functions to apply to the input data.
However, sometimes some additional functions are needed such as min, max or avg.
We need to find a way to specify, implement and document such extensions.

Start by examining the mapping utility and how jsonpath-ng functions are currently exposed. Determine how additional functions such as min, max, and avg should be specified, implemented, and documented; done means the extension approach and its documentation are in place.

Written by the indexing model from the issue text.