use the xdg base directory specification for home directory

[richardlehane]

By default vanilla siegfried defaults to a "siegfried" folder in users' home directories.

I should probably use https://xdgbasedirectoryspecification.com/ & on windows use appdata

An environment variable (SIEGFRIED_HOME) too would be nice. Order to choose home dir would be: -home flag first, then env var, then default location xdg.

[richardlehane]

feature merged in v1.11.0 release