Added XDG Base Directory support

[simos]

There is a recent go package that adds XDG Base Directory support to golang programs, https://github.com/zchee/go-xdgbasedir More about this at https://wiki.archlinux.org/index.php/XDG_Base_Directory_support

This pull request uses go-xgdbasedir in order to store the configuration file to the designated config dir of the system. The filename for the configuration is httplab.

Tested on Ubuntu.

[qustavo]

thanks for doing this, I'd like to create some specs to properly define/understand the behavior first before it gets too complicated