Change default install path to $XDG_DATA_HOME/zigup on non-Windows systems

Fixes #177. Also related to discussion in #146. Windows path is not changed because currently on Windows the path is derived from the location of the binary, so assuming Windows does not allow two identcal paths, the most logical path name, `zigup`, is not available. I don't know about Windows conventions and don't have a Windows machine available to be able to test my assumptions.