package/docopt-cpp: new package

Add docopt.cpp library that can be used to create intuitive command
line interfaces with very little code by describing the valid calls
using the (POSIX.1-2017) command line utility argument syntax.

Signed-off-by: Eero Aaltonen <eero.aaltonen@vaisala.com>
Reviewed-by: Angelo Compagnucci <angelo@amarulasolutions.com>
Tested-by: Angelo Compagnucci <angelo@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>