#81 Support XDG base directory specification

[Lucas Mior]

Files used / created by ngspice like .spiceinit and .ngspice_history end up cluttering $HOME.
There could be a way to save those files as described in the specification, maybe with a setting or environmental variable. In this case, .spiceinit would belong in $HOME/.config/ngspice/
and .ngspice_history would belong in $HOME/.local/share/ngspice