shell: dictionary commands feature

Adding dictionary commands concept. It simplifies creation of multiple
commands that are handled by a one function handler.

This is a special kind of static commands. Dictionary commands can be
used every time you want to use a pair: (string <-> corresponding data)
in a command handler. The string is usually a verbal description
of a given data. The idea is to use the string as a command syntax that
can be prompted by the shell and corresponding data can be used to
process the command.

Signed-off-by: Jakub Rzeszutko <jakub.rzeszutko@nordicsemi.no>