Dominic

Closes #134. I think that #144 should really be merged first since without it mach nominated version names will not appear in `zigup list`, making it unruly to use unless...

This PR add a build option `-Duse-xdg` that enables use of XDG_DATA_HOME for the installation directory on non-windows operating systems. This PR conflicts with #141 - whichever gets merged first...

add default-dir build option

5

Extracted from #139. This PR add a new build option `-Ddefault-dir` that sets the default installation directory. It defaults to "zig" which means that this PR does not change any...