nanocoap: add convenience function for adding path elements
nanocoap: add convenience function for adding path elements

Add a convenience function to make adding additional sub-path
options easier.

OTAkeys/RIOT,basilfx/RIOT,yogo1212/RIOT,OTAkeys/RIOT,jasonatran/RIOT,authmillenon/RIOT,miri64/RIOT,kYc0o/RIOT,OlegHahm/RIOT,authmillenon/RIOT,jasonatran/RIOT,kYc0o/RIOT,miri64/RIOT,yogo1212/RIOT,kaspar030/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,OlegHahm/RIOT,authmillenon/RIOT,authmillenon/RIOT,miri64/RIOT,RIOT-OS/RIOT,ant9000/RIOT,miri64/RIOT,ant9000/RIOT,miri64/RIOT,yogo1212/RIOT,kaspar030/RIOT,jasonatran/RIOT,yogo1212/RIOT,OTAkeys/RIOT,kYc0o/RIOT,ant9000/RIOT,basilfx/RIOT,ant9000/RIOT,basilfx/RIOT,basilfx/RIOT,authmillenon/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,ant9000/RIOT,kaspar030/RIOT,authmillenon/RIOT,kYc0o/RIOT,OTAkeys/RIOT,jasonatran/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,yogo1212/RIOT,basilfx/RIOT,kaspar030/RIOT,RIOT-OS/RIOT