Adopt XDG base directory specification

[pabloab]

Currently Miller place its customization file on $HOME/.mlrrc. Would be nice to follow the XDG Base Directory Specification. In particular, the config would be on $XDG_CONFIG_HOME, which by default is ~/.config.

$XDG_CONFIG_HOME defines the base directory relative to which user-specific configuration files should be stored.

The path would become something like be $XDG_CONFIG_HOME/miller/miller.yml (or .conf, if keep using current syntaxes instead of an arguably better/modern YAML, like for e.g. Alacritty).