This commit changes the default path of the Home Manager configuration
file from `~/.nixpkgs/home.nix` to `~/.config/nixpkgs/home.nix`. The
old path is still supported and will be used if the `.config` path
does not exist.
This aligns Home Manager with the preferred configuration directory in
NixOS 17.03.
Fixes#13.
This should reduce the risk of overwriting an existing file in the
user's home directory. A file will only be replaced if it is a link
pointing to a home-manager tree inside the Nix store.
If an existing file is detected an error is written indicating the
file's path and the activation will terminate before any mutation
occurs.
Fixes#6
