[core] add a datadir option to weboob init and handle WEBOOB_DATADIR
[core] add a datadir option to weboob init and handle WEBOOB_DATADIR

RouxRC/weboob,RouxRC/weboob,willprice/weboob,RouxRC/weboob,nojhan/weboob-devel,sputnick-dev/weboob,sputnick-dev/weboob,sputnick-dev/weboob,nojhan/weboob-devel,nojhan/weboob-devel,willprice/weboob,willprice/weboob