clients/ivi-shell-user-interface.c: Add 'command' config

Previously, commands could only be specified by their path,
so it was not possible to pass arguments.
Use the 'command' config first,
and if it is NULL, fall back to the existing 'path' config.

Signed-off-by: Gyeyoung Baek <gyeyoung976@gmail.com>

Co-authored-by: Michael Olbrich <m.olbrich@pengutronix.de>