Respect XDG_CONFIG_HOME if it is set

The changes here are trivial at best but I thought that it would be a good idea to fully support the [XDG Base Directory Specification] and therefore recognise when the system has set the `$XDG_CONFIG_HOME` environment variable. The codebase was already leaning that way by setting the configuration file location to `~/.config/pirate-get/pirate.cfg`, this patch allows the path to be `$XDG_CONFIG_HOME/pirate-get/pirate.cfg` if it exists, otherwise it falls back to the original.

If this is something you'd like but there's something wrong with the way I'm implementing it, then please let me know and I'll try to fix it. I wasn't sure where to put the `config_to_load()` function, or what to name it, really.

[XDG Base Directory Specification]: http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html